So how do we fit the Dal Quor into Planescape/mainstream D&D?

"Of course?" I don't agree with this statement at all. Eberron could easily be a Prime world accessible through Sigil's portals.

Dal Quor could be the same as the Region of Dreams/Curtain of Vaporous Color in the Ethereal Plane, or it could be a parallel Plane of Dreams from Eberron's cosmology (which might be separate from the Great Wheel, though I don't see much point in that - Eberron could easily be integrated into the Great Wheel cosmology, with certain Great Wheel planes more accessible on that world at different times; the planar orrery concept is already part of Planescape, as the fact that the Modron Orrery in Regulus is an accurate model of the multiverse makes clear).

My idea is that a portal to the quori kingdom in Dal Quor has only recently opened in Sigil, and quori have begun exploring the city, trading the stuff of dreams and establishing themselves while they search for portals leading to new worlds they could invade.

I wouldn't force the Inspired into Greyhawk or the Forgotten Realms. Keep them in Eberron and link Eberron to Planescape, or invent a new world to fit them in. Of the established Spelljammer worlds, Nivil from Practical Planetology is the easiest place to fit new cultures, since it's a ring world with a diameter equal to most worlds' orbits. But there's no reason you can't simply invent a new world that includes what you like about Eberron and throws out the things you don't think fit.

The fluff differences between Eberron and Greyhawk aren't any greater than those between Forgotten Realms and Greyhawk. Neither are its cosmological differences. Eberron is much less a "closed world" than Athas. The technological differences are irrelevant as long as traffic between Eberron and other worlds isn't common, and pose less difficulty than the economic differences between Toril and Krynn. Sigil can link to anywhere, in any cosmology, regardless. I don't know why Nivil's thin shape would prevent it from having Inspired in it.

There's no reason not to make Eberron accessible from Sigil regardless of how you interpret its cosmology, but assuming Eberron is a world in the Great Wheel with an unusual relationship with surrounding planes wouldn't change play in the world.

There's not much to say about the Modron Orrery except that it's an orrery that manages to be an accurate model of the Great Wheel. There are lots of references in obscure canon to planes being closer or further at various times in relationship to celestial events. Eberron has better mechanics for this, is all.

On a side note, Mystara has more official portals leading to other D&D worlds than any other, mostly found on the conversion pages of the Gazetteer series, though there is also a Spelljammer connection between Karameikos and the Rock of Bral in the First Quest boxed set, making spelljamming from Mystara to any named wildspace locatiom possible.

There are problems with connecting any campaign world with any other. Most worlds have their own origin stories for various monsters, differing levels of technology, and cosmological assumptions. Eberron isn't unusual in this, and it's a lot closer to standard D&D than many.

The monsters in Greyhawk and Forgotten Realms don't have the same origins, is my point. Mainly, this is because the Forgotten Realms is a very detailed setting that gives specific origins for many monsters, in a specific place and time on that planet. For example, the drow of Faerun split from the rest of the elven race during the Crown Wars, which is given a detailed history in Faerun - so where does that leave Greyhawk's drow? Did they originate in Faerun and migrate to Oerth? It's because their origins of dark elves in both worlds are similar, but not exactly the same, that it poses such a difficulty - they're clearly the same race, but where'd they come from? Did they appear independently, in two separate wars, or did the Crown Wars spread to multiple worlds? The drow of Eberron, with their history of giant subjugation and completely different religion, are much easier to conceive of as a completely separate elven race that only shares a dark skin tone with the drow of Oerth and Toril.

There are many substantial differences between how creature origins are handled on Faerun and Greyhawk, though. Yuan-ti on Oerth are descendants of Olman humans who made a pact with the god Tlaloc, while on Toril they're creations of the sarrukh. The duergar of Oerth are assumed to have originated when the One Clan lost the Axe of the Dwarvish Lords in prehistory, while in Toril they were enslaved by the illithids of Oryndoll. The derro of Oerth were hybrids of enslaved dwarves and Suel humans created by the ancient Suloise, while the derro of Toril are thought to have been experiments by mind flayers breeding humans and duergar. The skulks of Oerth were members of the Suel undercaste transformed by Suloise wizards, while the skulks of Toril were created by drow.

There are many examples of things like that between Oerth and Toril, origin stories where two clearly identical creatures have contradictory origins. Maybe it's parallel evolution. Maybe all of these creatures originated on Toril and migrated to Oerth, or vice versa (or came from other, unknown worlds). In that case, the official fluff is simply wrong - but you can just as easily declare Eberron's fluff to be wrong.

Eberron's fluff is generally easier to resolve with Greyhawk or Toril's fluff, I think. For example, in Eberron beholders came from Xoriat, while in Greyhawk and Toril they're spelljamming ignorants who are creations of the Great Mother, who currently lives in the Abyss but may have originated in the Far Realm (which is mostly identical to Xoriat - and yes, in this case the origin fluff is identical in Toril and Greyhawk). So maybe some of them traveled to the Far Realm/Xoriat and migrated to Eberron from there. Problem solved. The illithids may have done the same thing, some of them migrating to Xoriat and becoming enslaved by the Daelkyr before settling on Eberron.

Eberron was designed with the idea that nearly all 3rd edition material could be easily used with it. Its way of handling religion is a case in point - the gods of Eberron are known by many different, sometimes obscure names, and in some cases multiple local deities were "merged" to form the members of the Sovereign Host and the Dark Six. So, for example, Balinor may be known as Obad-hai in some places, and Dol Arrah may be known as Heironeous, and Onatar may be known as Moradin. The Mockery may be also known as Hextor or Gruumsh. But really, it's not a problem for people to worship entirely different gods in different worlds, so I'd rather keep them separate. The fact that the elves and dwarves of Toril and Oerth worship mostly the same set of gods (with some differences) lends support to the idea that they have a common origin, which as I said earlier is something of a problem given the very Toril-centric history of the elves and dwarves in Toril. If the Crown Wars and the exile of the duergar had happened in the obscure past, in an unknown place, reconciling them with the history of those peoples on Oerth wouldn't pose a difficulty. The different history and theology of the dwarves and elves on Eberron poses less of a problem in that respect - you don't have the dilemma of the same essential events happening in two different places. If the elves of Toril and Eberron have a common origin, it's long before the Crown Wars.

I take your point about Nivil. I assumed you were just going to drop the Inspired on a world with room for that culture (which Nivil has plenty of), rather than drop their entire continent on another world. You could possibly fit their continent on some obscure and ill-defined official world, like the World of the Red Forest mentioned in Expedition to the Demonweb Pits, or super-obscure worlds like Tuimuhva (Tales from the Infinite Staircase, 19), Verdoth (PSMC3, 96), Glemayne (Ombidias's homeworld from the Factol's Manifesto), Neirston (Dead Gods, 12), Rublia (Uncaged, 120, and The Factol's Manifesto, 83), Gallamede (from Fiendish Codex I: The Lost Entries on the WotC website), Talaron (home of the Talaire from Complete Psion), Malena (from Conan the Marauder, a 1st edition AD&D adventure that involved traveling from Hyborean Earth to a parallel world), Tir or Masg (from Dragon #244), the 4e core world, or the world of the Jakandor setting.

Whether it works better for you to just use Eberron (as I would), to use a world that's already partly detailed or at least mentioned (possibly one of the worlds listed above), or to invent an entirely new world is, well, up to you, of course.

Mystara poses something of a problem because it originated as part of a slightly different game system, although many of its differences with standard 2nd edition AD&D were ironed out when it was transformed into a 2nd edition AD&D world. A lot depends on which version of Mystara you use. Much that is distinctively Mystara could be retained in a version where its Immortals are standard D&D deities and its planes the standard D&D planes, which several 2e products assumed was the case. That said, as is, I think, the case with Eberron, you could retain its cosmology and distinctive tropes in full and simply assume it's a very different reality that Sigil, the World Serpent Inn (which is an official way to travel between Eberron, Oerth, Toril, Athas, Sigil, the gate-town of Ecstasy, and Krynn, according to Dragon #351). There's a portal to Sigil in the Urban Arcana world for d20 Modern, after all, and you don't get much more isolated and different from D&D standard than that.

I think Krynn is much worse than Birthright (and worse than Eberron) when it comes to ease of crossovers. Krynn's iron-based currency is the hardest part - it's too easy for worldwalkers with a two-way portal to bring iron from other worlds, trade it for valuable goods, and keep doing that until they've destroyed the local Krynnish economy with inflation. That's not any worse than Athas, though, with its lack of iron weapons and powerful locals. Mystara is a typical medieval economy, and a comparative piece of cake - there's nothing they can bring from Oerth, short of a dangerous artifact, that will cause much havoc on Mystara. Torilians might cause a stir with smoke powder weapons, but Mystara's Savage Coast already has smoke powder (in 2e, anyway; it didn't in basic D&D).

Eberron's artificers and magewrights might cause problems on other worlds, or they might not, depending on whether or not they emigrate en masse and decide to share their knowledge. It depends on whether it actually matters to you if Oerth or Toril get elemental trains or not (you might be fine with it), and if things like magecraft spells even work on Oerth or Toril (magic often varies from sphere to sphere). Keeping portals rare, temporary, and usually one-way keeps it from being a problem regardless. Most Eberron features (quori, magewrights, inspired, zakya rakshasa, magebred animals, warforged, dusk hags, homunculi, daelkyr, etc.) could be imported to Sigil without any problems.

More evidence that the Great Wheel is an orrery includes Dragon #151, which features a yuan-ti brew that can only be created when a certain Abyssal layer is in conjunction with the Prime every thousand years; the periodic cycle of certain portals, such as adventure Gates of Firestorm Peak, which features a portal to the Far Realm that opens periodically, and the portals of Tovag Baragu on Oerth; the late-2e adventure Return of the Eight, in which the witch-queen Iggwilv can only open a gate unleashing an Abyssal army on a certain date; and the (tragically noncanonical) Outlands calendar on Mimir.net, in which different Outer Planes influence Outlands weather at different times of the year. The main problem is the lack of consistent rules for judging when planes of the Wheel are closer or further from other planes. Eberron (and certain Abyssal layers) may have a more extreme orbit through the orrery than other worlds and planes.

My main contention is that you don't have to reconcile cosmology and lore on all the worlds in your campaign. The players, for the most part, won't know what's true, probably won't care terribly much, and may not even hear all the varying stories. As a DM it may be useful to have a single overarching mythos. I think it's possible to build one that accounts for all the wildly varying worlds, but it may make more sense to create one that ignores the "official" versions of things in favor of what works best for you.

I don't want to discourage you from inventing a semi-Eberron that has Inspired but not artificers or Daelkyr if that's what you want, but I think it's equally valid to use a world that's almost identical to Eberron except that uses the Great Wheel cosmology, or a world that's identical to Eberron in every respect but happens to have a portal to Sigil that the quori are using to escape.

Yes, there are examples of that happening (for example, dwarves, elves, fey, halflings, gnomes, illithids, beholders, humans, orcs). This is not true for the examples I gave, however, and not many other examples as well. Drow, duergar, derro, yuan-ti, nagas, bullywugs, dopplegangers, kobolds, skulks, aranea, aarakokra, and other races are explicitly native to Toril, which makes it somewhat difficult to figure out how Toril and its races fit into a cross-world campaign. The scaly races were mostly creations of the Sarrukh, kobolds were created by the green dragon Caesinsjach, dopplegangers and bullywugs were creations of the Batrachi, aarakokra were descendants of the Aeree, skulks and aranea were creations of the drow, duergar and derro were "created" by the illithids, and so on.

The Ssri-Tel-Quessir (the ancestors of the drow of Toril) were not drow; they had brown skin and dark hair rather than the ebony black skin and white hair of the modern drow race, who are tainted with the blood of the balor Wendonai during the Fourth Crown War. Drow on other worlds would either have to be descended from Torilian drow or be the result of a similar event on other worlds. Core/Greyhawk versions of the story don't mention a dark-skinned ancestral race (the implication is that they were once the same people as surface elves), so Greyhawk's drow might not be related to the Ssri-Tel-Quessir at all.

Saurials are from a parallel world, and brought to Toril by Moander. They're not indigenous to Realmspace.

Anyway, I don't want to discourage you from creating an Eberron-like world made to fit more smoothly with Planescape if that's what you want to do. Personally, though, I'd just use Eberron, because I don't think it's harder to fit into cross-world campaigns than the Forgotten Realms is. There are certainly ways that Forgotten Realms canon can be reconciled with Greyhawk canon (or, of course, we can just ignore canon where it's inconvenient), but that's just as true of Eberron's differences. I think the fact that Eberron usually has a unique take on nonhuman cultures makes it easier to reconcile, not harder, because we don't have to assume that parallel evolution (or creation) resulted in nigh-identical groups.

I aloud a character in my party to play The Planar Shepard out of Eberron Campaign Settings. First off, the planar Shepard from planes scape covers this b.s. class.

As far as the plane of dreams? Well look at this way everyone dreams. It isn't outside the realm of possibility that he some how "planar shifted" in Faerun. If you look at the map of dream scape it is quit possible to do this as the dream world is created from "mass imagination".

I tend to allow the following classes and races into Forgotten realms for d&d magic the gathering.(this is what I call Eberron because if you look through all the b.s. it is magic the gathering in a d&d setting.)

Races: Warforged(created in lantan), Shifter, Changling

Classes: Artificer

These are pretty much the only classes and races that translate well to Faerun.

Oh and Ravenloft is not on the plane of shadows or the demi plane of shadows, it resides in it's own domain called the Domain of Dread(read 2nd edition ravenloft, particular domains of dread)