Okay, here's the ideal incarnation of this setting concept as I see it right now.
The World, Pre-Invasion
No demihumans, fey, dragons, magical beasts, oozes, or undead. I kind of want to say "no giants", too, but I think there's room in this setting for ogres and trolls (although, really, ain't they more like monstrous humanoids?). One of the main points of these choices is to remove blatantly magical elements from the world. (I didn't even bother to mention outsiders, elementals, and constructs, since they're pretty much not part of the local ecology
by definition.)
The main intelligent races are humans, orcs, goblinoids (which are one race with ant-style variant castes), and maybe lizardfolk and gnolls (I generally abhor the whole humanoid-animal-as-fantasy-race bit, but these two ain't bad). I'd really like them all to be playable races, even if that takes some mechanical screwing around. Pretty much the whole damned world is fairly savage (neolithic, to broze age at best), but there are a few isolated--and isolationist--human groups that are more advanced. Also, the goblinoids once had a considerably more advanced empire, but it fell apart millenia ago. (Is that too Eberron? My first instinct was to say the lizardfolk used to be more advanced, but that would be recycling an older cliche.)
The handful of advanced human cultures are the only ones who've learned the secrets of arcane magic. (In fact, maybe that's their only advancement--aside from a more civilized culture--and they don't have steel or even iron. Maybe the orcs or somebody have that secret.) The rest of the world uses divine magic, primarily in the form of the
spontaneous-casting Cleric variant. (Obviously, the Turn/Rebuke Undead class feature needs to be swapped out for something else.) The "gods" are thousands of squabbling spirits who are generally known only to single tribes. Psionics are completely unknown.
The Invasion
It starts in the swamps and the caves: Tiny holes are scratched through from someplace just as dark and damp, and the wormish larvae of the chuul are pushed through by the hundreds. Most of these die in the strange new environment, or are consumed by terrestrial predators, but those who survive grow huge and powerful enough to dominate their habitats. Then they receive their instructions, transmitted psionically from their home dimension.
The chuul who are psionically sensitive enough to get the messages begin work on more substantial gateways. (I'm picturing the "plague of frogs" from the comic
B.P.R.D., in which fast-reproducing frog people spread quietly across the U.S., building terrible altars and conducting blood sacrifices as they go.) Gradually, more alien horrors are able to emerge, including both active invaders and assorted alien animals.
I like the idea of using a wide variety of different aberrations and oozes for the invaders, and a few of the weirder magical beasts, as well (stirges, ropers, etc.). Any magical abilities they've got should be recast as psionic ones (if that makes any difference). (Note that the psionics rules
are in the SRD, so they're fair game for this project.) I even kind of like the idea of recasting them all as a new creature type, "alien". That way, we could use that type as a replacement for alignment, and have abilities like "Smite Alien" and "Detect Alien".
Illithids, beholders, and aboleths are the clear candidates for the masterminds behind the whole invasion. I've got a soft spot for the aboleths, myself. Beholders aren't traditionally seen as capable of cooperation, but I don't think it's necessary to retain that element of their character. Mind flayers are obviously great for this whole role, but I don't like the fact that their bodies are so humanoid. Maybe that's just how they end up looking if they hatch from a humanoid host?
Anyway, whoever's running the show, they and most of their servants are dependant upon a particular type of environment. They like the darkness, and they like moisture, and they prefer a particularly noxious kind of atmosphere (and water, for that matter) that is inimical to terrestrial life. So as they take over territory, they construct psionic artifacts that gradually transform the surrounding region into something approximating their poisonous homeworld. Maybe they (inadvertantly, even) bring some terrible new diseases along with them, too.
The Response
The subterranean races notice the alien encroachment first, and end up fighting among themselves for space as the taint spreads through their caverns. Eventually, they're pushed to the surface, and make nocturnal raids against the other races to obtain resources and new homes.
By the time the more advanced cultures realize what's happening, the invaders have gotten more than just a foothold, and the subterranean races have been all but completely evicted. Even places the invaders haven't actively made any move towards begin to suffer from the tainted atmosphere of the changing world.
Still, the isolated and advanced human cities stay out of the brewing conflict, hoping that the invaders will be content with the dark places of the world, and leave the surface alone. Then one of their own settlements falls to a combined force of hardier aliens and mentally-dominated humanoids, and is hidden in the endless shadow of an unnatural toxic cloud, and the civilized people are forced to accept that this is their problem, too.
Now, the various races and cultures of the world must work together, sharing magical and technological advances they once kept secret--and social advances that their neighbors don't want to hear about--in order to build a new civilization capable of resisting the invaders. Inevitably, though, there'll be as many clashes between the defenders as with the aliens themselves, and getting very different peoples--in many cases, plagued by historical animosities--to cooperate or even coexist in the shrinking territory that remains to them will be a major struggle.
The advanced cultures share arcane magic, the various savage cultures share divine magic and possibly iron or steel, and some knowledge of psionics might even be stolen from the invaders.