Ninja and other classes who can go ethereal

No - you're right on that. But one of the proposed solutions for GM's in this regard is to give Sigil it's own Astral and Ethereal overlaps, or at least overlaps that act very simular to the real thing - *also* locked like the way Sigil's physical space is locked from outside interference. That would allow the use of some of the spells folks want - just only within the microcosim that is Sigil.

Argh, Manual of the Planes!!!

Astral and Ethereal do not overlap. They are mutually exclusive, except on the Material Plane, which is their one and only point of contact. Sigil, being exceptional in many ways, also has portals leading to both of them, but otherwise... Astral for Outer Planes and Ethereal for Inner Planes. Plane of Shadow used to be considered just a demiplane, but I think the current consensus is that it's a state of being or somesuch. Not sure with that one.

Anyone else consider Sigil to be one immense and ancient Demiplane? Or is it just me? Explanation for my rambling blasphemy on request.

B
xxx

Nah, I just thought controlled methods of entry/exit, finite borders, ruled by one 'person' who defines planar traits. All features of a demiplane. Lady of Pain is powerful...maybe she shifted it to the center while everyone else was busy sorting the other planes, or maybe the other powers put it there because it went nowhere else.

If it was unwanted or not meant to be there, that would explain the idea I read somewhere that the Spire is the result of Sigil being pushed off the Wheel. Kinda like a planar disease being cured by a planar immune system.

That';s essentially what we're suggesting too. Just don't let someone pull the "I go astral then go... someplace *else*" routine.

... what happens in the Cage... *stays* in the Cage...

I'm not saying we ignore the MotP, I say we filter it. It wasn't designed to convert all of Planescape to 3e, it simply took a lot of ideas from it and added them to 3e. However, some things obviously don't match. One of those things is the ubiquity of the Astral. It wasn't so before, and quite frankly, despite Sigil spell and all, there should be no reason for it to be so now. The Plane of Shadow is a lesser point... there is the possibility that a demiplane evolved into a Transitive, but again, there has to be a reason. Why is there suddenly enough belief in it for it to evolve? All that I'm saying is that just because WotC is the official publisher of D&D3e (and now 3.5e), we should not treat their books as the ultimate authority on Planescape (unless they wish to truly resurrect the franchise, and give in game reasons for changes). The new rules for how much damage you take on the plane of fire? Sure... it makes sense, and its balanced with the d20 rules. The new idea that modrons are replaced by formians (which are now from Mechanus instead of Arcadia)? Not without a reason.

Okay, I'll step off my podium now.

Depends. If you have the 3.5 DMG, that covers basic planar descriptions of the Great Wheel. I'd suspect MotP has more detail in it on said planes, and includes all kinds of other useful information.

Just the original PSCS box? Probably not. Although many other PS materials would be of great use, I'm sure.

The plane of thought - the backdoor of the realms. Imagine a space - now half that space, half it again, and again, and again till there is nothing left... well - That's where the Astral is. It is a plane of pure concept, no time, no physical form, just thought and the mind's power. It is not a place that mortals, primes, were ever truly meant to be.

As a transitive plane it overlaps the Prime and the Outer Planes. It is through the astral that thought travels, to create the Outer Planes. It is through astral conduits that spells and souls migrate to the Outer. It is where discarded memories of the dead consolidate before winking out like ever so many crystalline stars, and where the grand concepts - the gods - die when they are abandoned.

Here's the best way to handle it...

Does your game gain or lose anything because characters can go Ethereal in Sigil or the Outer Planes?

If you think it's lame that they lose a class feature, then simply have it something that they can do.  Perhaps it's a loophole, or something unexplained, or perhaps there is an Ethereal Reflection (or 'layer') of Sigil, which I already wrote about in an article on this site.

Well, if you go by Planescape Torment then Sigil really should have its own plane of shadow. You can meet Shadows (the monster kind) in Sigil, and if I'm not mistaken they need a link to the plane of shadow just like ghosts need the ethereal.

 I think I read somewhere also that while you can go to Sigil in the astral, you can't enter it. There's some form of barrier that stops you from entering the city, even if you can float around just outside it in the astral, if you want to. Similarly nothing can leave Sigil through the astral, being behind the impenetrable barrier.

 Though, Sigil being Sigil there's really no reason why you couldn't find a Portal inside the Sigil Astral that go somewhere else, just like you can find everywhere. The reverse could also be true, which could lead to a frustrating situation if a hapless planewalker somewhere enter a Portal expecting to enter Sigil and ending up in the astral version of the city instead.

Personally I would also give Sigil its own ethereal demiplane, if nothing else for ease of play. The plane would be cut of from everything but Sigil itself, but it would allow people to use ethereal powers and such, plus the above-mentioned ghosts. (Torment once again shows us ghosts in Sigil so it should be possible, but on the other hand one of those ghosts is somehow able to go to another plane which makes me wonder if the spirits of the dead are somehow exempt to the normal rules of the cage, or maybe there's a portal on the Sigil Ethereal that allows exit in a manner similar to the one I theorized about the Sigil Astral above).