Adventure Path on the Elemental Planes

I just finished reading “Secrets of the Lamp” involving the various genie races. So with some inspiration from there, here is my (weak) stab at an “Elemental Adventure Path”

-While on some side adventure to the Ethereal, a PC comes into contact with some proto-matter and now a part of his body has reverted to proto-matter (describe the “unknown substance” in such a way as to make the proto-mater enticing so that the PC starts poking at it by his own choice). Now, he has to find the means to reverse the condition or else his arm with dissolve when he returns to the Prime
-First trek is to find a scholar (residing in the Ethereal perhaps near a “gap”) who will then explain that the PC must trek to the four elemental planes to restore the individual. Also says that the PC will be fine so long as he remains on the Ethereal or one of the Elemental Planes (perhaps the affected body part will transform resulting in a arm of solid fire while on Fire, an arm of stone on Earth, etc.) but must have the “quintessence” of each of the elements to be whole back on the Prime
-Scholar suggests the second trek to the Plane of Water to obtain a magical means for the party survive on all the elemental planes (the scholar has an inferior magic to allow them to breathe on Water for a while). Minor adventure exploring Water while searching for the marid
-In exchange for the magic to survive the planes, PCs are asked to investigate rumors of an artifact remaining within the ruins of the Elemental Evil of Water. Because of its properties, it is rumored to be especially “damaging” to the marid. The marid potentate (with great difficult) asks for the PCs help to retrieve it so that it can be destroyed. After retrieving it (in a special container), they are given the quintessence of Water they need
-Since it is uncertain to which plane the PCs will go next, the rest of these suggestions have to be flexible. Most likely, the PCs will take a trip to Air next but if they go to Earth, they find the dao, elementals and other natives even more reclusive and secretive than usual. They will make no progress on Earth at present. Behind the scenes, a mysterious figure has promised the dao freedom from the yakmen in exchange for some covert acts.
- If the PCs go to Air or Fire: the dao were told by their mysterious benefactor to kidnap a djinn (or an efreet) but make it look like it was done by an efreet (or a djinn) in an attempt to start an inter-planar war. PCs get involved to solve the mystery of who was really responsible. The PCs will get the quintessence of the current plane (Air or Fire) for solving this mystery
- While the PCs were unraveling the mystery, some angry young djinn (or efreet) took a band of his men (and related elementals) for military actions against their “enemies”. Now the PCs are now asked to find the Sultan of the Efreet (or the Caliph of the Djinn) to try to act as ambassadors to try to prevent the rash acts from preceding event from starting a war; but the ruler is out on a dangerous hunt. PC must travel through area with dangerous predators to reach the ruler. Despite their best efforts, they aren’t believed and the conflict starts to escalate (although the PCs will receive the elemental component they need from the ruler who was impressed with their bravery)
-A water gen or other friend from the earlier adventure on Water tells the PCs that the artifact retrieved in the earlier adventure from Water HAS “damaged” a marid court in that it has corrupted the PC’s former allies to evil. Now there is a civil war among the marid. PCs must deal with attacks from the good marid (as the PCs are known friends of the now evil marid) and with trying to find a way to defeat their former friends while killing/injuring as few as possible. Even after the PCs fetch the evil artifact, this has no effect on the current conflicts in Water which are being aggravated by hidden agents
-Trek into the Plane of Fire to destroy the evil artifact of water. Deal with agents of Elemental Evil who want the item back
-Despite the PCs’ best efforts, war is escalating in Water (civil war) and between Air and Fire. PCs will probably just want to get their last element (Earth) and leave. Upon arriving on that plane, the PCs are surprised to find the plane in chaos as a growing number of dao have joined a new faction that has thrown off the influence of the yakmen. This faction is rapidly growing in power and a LOT of violence is happening on Earth (mostly focused on the yakmen but PCs are probably close enough in physiology for the dao's rage). PCs will find that the center of this violent faction is a prince of Elemental Evil seeking to rebuild his cult
-Incriminating evidence found indicating that all of the conflict on the Elemental Planes has been due to the agents of Elemental Evil trying to rebuild their lost power. Implications that if they are not stopped now then their next point of attack will be the Prime

I think the last stage(s) should involve more of the elementals and less of the genie races; but I’ll leave this higher level stuff to others for the time being

Oh, and to Palomides: I like your idea. However, I would expand upon it: The party should include a true-neutral druid and a priest of one of the elemental lords (thus having the domain of Balance), and the Princes of Elemental Good are also meddling, hence the civil war (they have taken the opportunity to try and convert the genies to good in order to offset the evil, but they have also gotten ahead of themselves and want to turn all the genies good) The quest may also involve dealing with the Qorrashi genies on Ice, and allying with Crystalle, the Prince of Quasielemental Neutral Mineral, who is worried about the balance of the planes. Working with the elemental servants, druid circles, and priesthoods of the Elemental Lords would be a must.

I don't think low-level adventures are even possible except within the confines of some Air and Water cities, and even then, you'd have to really go out of your way to plot-device the whole thing so that stronger creatures don't get involved.

Even the City of Brass isn't a place for low-level characters (sure, they can survive there, but one false move and a horde of efreet will be opening a can of flaming whoop-ass on the party. Ditto if your players poke around at stuff too much-- the City of Brass is full of crap that can burn them to death and then some)

One option however is to send the characters to a city that is racially segregated, and have the adventure take place almost entirely in the Mephit Quarter >_>
Though if you want to go a much more boring, less exotic route, just have it take place within the confines of a human/elven/dwarven/etc. colony city or town and make it a cityscape quest (yaaawn I hate cities-- both in gaming and IRL)

As for the adventure path I suggested, the initial part (planetouched sacrifices and kidnappings) could or even should take place primarily in two or three dungeons on the Prime, so the players will be raising their levels a bit, there.

The idea the PCs shouldn't try to fight every "monster" they encounter and instead find other ways to deal with them, is pretty central to Planescape.
There's more to it than that, though. Say or do the wrong thing, and the Efreet are likely to attack you. Especially since, like all genies, they are quite capricious.
On Sigil you can ends up in areas or buildings inhabited only by low-level creatures.

City of Glass on Water could be an interesting setting. It may not have the immediate recognition that the CoB does, but it's certainly a friendlier city to any visiting PCs.

There's another one that was written up on the boards here, set on Smoke that had some interesting themes going on. I don't recall what it was called though.

Set it against the backdrop of the ongoing war between the Efreet and Djinn. The Efreet have been hiring mercenaries from Baator and have been paying for them with slaves. Getting the PCs to stop or disrupt the negotiations between the two groups could feature into one of the adventure points. If you want to avoid Inner/Outer planar mix-ups, then you could change it into Efreet and Dao.

I suppose a good starting point would be: What kind of adventure path do you want this to be? Something epic that spans the Inner Planes? Something that showcases each of the planes in turn? Do you want to delve into the various political rivalries that exist on the planes (between the genie races, between the opposite elements, between natives and outsiders)? Maybe you want to touch on all of these aspects, in which case, making the AP into a chase or a race against time would keep the PCs from lingering on any one plane for too long. Or maybe you want it confined to just one or two planes, which would change the whole tone and pace of the AP.

You've already said that you want the PCs to be low level'ish, but do you want them to be planar or prime (or a mix of both)? Each one leads to a much different feel for the game in terms of prior exposure. My own personal inclination would be to make the PCs all prime and use this adventure path as a means of introducing new players to the Inner Planes aspect of Planescape.

The idea of a group trying to "break" the City of Glass is somewhat intriguing to me although it seems a little grand for lower-level PCs (Damn! I just broke my own rule - no censoring!) For lower level PCs they might get wrapped up in some preliminary stage of this grand plot. Perhaps there is some intrigue involving a race of "harmonics" that communicate in tones like tuning forks (is there anything like this now? if not, might they be residents of the Plane of Crystal/Mineral?) Maybe some group is kidnapping a number of these beings for unknown reasons.
Or it could be an inadvertent infestation of tiny glass-eating vermin. That one seems a little more in-tune for low level characters.

What's the relationship between the Inner Planes and the Prime Material? I've always understood it as a metaphysical thing: the Inner Planes represent the building blocks of all matter in the 'verse. They're more than just that, of course. Elementals have intellect, societies, religion (of a sort), and so on, making them more than just the sum of their particulate parts. But my thought for an adventure path through the Inner Planes hinges on my question.

So here's my thoughts...

The players tumble across a world egg. It's quite possibly the last of its kind. And seeing as how there aren't any new worlds (or crystal spheres, depending on how you depict the Prime Material) being made these days, it stands to reason that a quite a few bloods would be very interested in getting their hands on such a rare possession.

What do the PCs do with their treasure? Nothing unless they know what it is. Naturally someone has to tell them, and it may seem a bit like railroading, but that can be avoided by giving them ample opportunity to turn to a different path. So some ancient basher gives them the dark of the egg. They know they can't do anything with it (because they don't possess the knowledge and/or magic to hatch it), and they know that, should anyone find out they have it, someone powerful's gonna come looking for it. Perhaps they get a bit of information from the basher before signs the dead book? Perhaps there's a power interested in the egg, but doesn't want his/her/its rivals knowing about it, and uses subtlety and subterfuge to inform the PCs. Either way, now they know what it is, what it can be used for, and that they can't use it unless they learn the right rituals, or give it to someone who knows them.

This is where the Inner Planes come in: the egg is the unhatched potential for an entire universe of Prime Material worlds. But it needs a bit of raw creation in order to hatch properly. The PCs have to travel to the Inner Planes and retrieve some; but it can't just be your everyday, elemental matter. No, what they need is pure Water, from the very core of the plane itself. Same for the other three; maybe they need more than the traditional elements, like maybe positive and negative energy, in order to give their egg life and death. Maybe they can substitute one of the para- or quasi-elemental matters, though that may skew the evolution of the egg. And maybe the pure elemental matter is something that each archomental guards; maybe that's the reason the archomentals exist; maybe the pure matter is a literal part of their body.

Catalyst: the PCs acquire a world egg, an unhatched universe ripe with potential.
Goal: find pieces of pure elemental matter in order to hatch the egg.
Motivation: whatever the PCs want it to be. They could give the egg over to their god(s), who return it to them with their mission. They could seek to create a universe of their own, which they can shape and mold however they choose. They could decide to auction it to the highest bidder, though without the ritual for hatching it...

And seeing as how there aren't any new worlds (or crystal spheres, depending on how you depict the Prime Material) being made these days, it stands to reason that a quite a few bloods would be very interested in getting their hands on such a rare possession.

Is this mentioned somewhere in the canon? Because I find it highly unlikely, especially due to its limitations to the DM.

It would limit the DM by basically saying that they can't have the PCs engage in an adventure that results in the creation of a crystal sphere if they want to remain true to the canon.

So basically, your adventure suggestion would only work within your own homebrew and would not be compatible with the canon.

I don't think cannon matters (and really it never did) - customize the planes as you wish. You just need to keep in mind that the more you customize something then the more you are speaking your own language.
I have a number of customizations that I like and frequently, they would fit it with or resolve some issue brought up on a thread. But I have to step back and remind myself that my ideas aren't common knowledge so I can't build up some elaborate thing based on my own system and expect anyone to care.
In the interests of sharing, we have to start from the commonly accepted baseline each time. But for throwing out new ideas within a single thread, I try not to shoot anyone down unless the non-cannon idea leads to some MAJOR complications (e.g. gods can disobey the Lady of Pain and enter Sigil)

I'm not a "he". I try to stick to canon, yes. You can do what you want in your own campaign of course, and you can house-rule whatever you want both fluff and crunch-wise. However, these boards are about Planescape, with the canon in mind. As for being a stickler for the canon, I also write fanfiction, and I do my best to stick to canon and keeping things in-character-- I find it more fun that way, to operate with some constraints. Plus, it's fun to plot-device as to why things are the way they are.

Also, I SHOULD clarify that there are very rare times where I disregard or contradict the canon-- I only do this when I think the canon is stupid, nonsensical/illogical, or poorly thought out (usually all three) I can't recall any examples off of my head other than renaming Coldfire "Coldflow" for several reasons (one being that there are already so many different things by that name in the canon, and more importantly, because the natives of Ice would NEVER refer to it as that-- the very word "Fire" would be offensive to them-- that said, I have it in my writings that green primes refer to it as "coldfire", but they'd sure as hell never refer to it as that around Ice or even Air/Water/Mist natives-- they regard that as blasphemy) However, that really isn't a disregard or contradiction of the canon--
Wait, I do recall an instance where I disregard the canon-- the whole Protogenoi thing from Greco-Roman mythology. For instance, Gaea is a Protogena, and NOT a Titan. Also, Charon is a 'loth, but was born from the Protogena known as Nyx.
How can you tell if a being from Greco-Roman mythology is a Protogenos or a Titan? Simple. Is it depicted as a person, or as a personification of a natural landmass or the like? (For instance, Gaea is depicted as the earth itself. Zeus even impregnated her at one point by getting drunk, passing out in a barren plain, and having a wet dream. Seriously. I think there was a similar instance in Egyptian mythology where one of the gods impregnated the earth by humping a hole in the ground, LOL. Erebus is depicted as a mass of darkness in Tartarus-- also a protogenos. Nyx is a Protogena even though she is sometimes depicted as a person.) In my writings, the Protogenoi also encompass elemental-like beings from other mythologies as well, such as the hermaphroditic Ogdoad from Egyptian myth, and Chauntea, Shar, and Selune-- along with former incarnations of Amaunator from the Torilian pantheon. Many of the beings in animistic/shamanistic/tribal pantheons are Protogenoi as well (e.g. the sun from most Amerindian mythologies, or East Wind and the volcanoes from Chinookan myth)
Admittedly, even with the true titans things can get murky, as many of the titans are also depicted as landmasses. The best way to tell is to look them up on Theoi.com-- you should take the Theogeny and other Hesiod works over other sources (they tend to be the most classic source and rank among the oldest of the sources-- later sources tended to merge several deities into one, such as Selene and Artemis)
I just though of another area-- albeit minor-- where I contradict the canon-- Uranos's reason for hating his children was not because they were hideous with their many eyes/hands/whatever, but because they were repulsive beings of flesh (some elementals find fleshed ones to be repulsive-- Ooze Paraelementals never do of course.)