I oppose this on principle as well. The simple fact, every dwarf knows, is that the moon up there and all its other strata spheric friends, were all mastercrafts made by a legendary dwarf cheese maker in a farm. Hence, anything originating from those moons must be dwarfskind, not mankind.
You're almost right. All of reality is in fact a 9 layer cake wrought by the finest cakesmith in the known universe who, in a fit of raw creation, baked a cake so supremely magnificent that the universe itself felt a bit shabby by comparison and swapped places with it.
I still say they were made out of stone and metal! And that things vaguely like spacemen in concept, and able to mod in actual spacemen, are good ideas.
What about mysterious underground civs putting outposts on the surface? Like a drow raiding party (To use a somewhat stereotypical example) or something like that.
I Like that idea, personally... Maybe not... Drow, but more ancient things.. that aren't elves. Say maybe corrupted Dwarves. After all, elves can't dig, even dark elves. They just make wooden things out of magic. and carry elephants on donkeys. (and probably released the elephants on boatmurdered) Or something else entirely.
Hm, how about...svirneblin, or deugar, or mind flayers, or deep halflings (halflings with some dwarven traits, thus being like a combination of elf and dwarf but shorter), or (locates several D&D books) chokers (small, humanoid flexible aberrations that are VERY fast [old-edition haste]), or derros (small, insane dwarves), or driders (half-spider, half-drow, basically--it's complicated), ethereal filtchers (four-armed one-legged body-faced aberrations) with or without the ethereal, or ettercaps (spidermen, spare me the puns), or formians (ant-outsiders), or gargoyles, or some interpretation of ghouls, or gnolls (hyena-men), or grimlocks (eyeless humanoids with blindsight), or hobgoblins, or kobolds (oh, wait), or magmins (humanoid fire elementals), or orcs, or rakshasas (magical tiger-people), or skum (subterranean fish-people...things...more or less), or troglodytes (the D&D kind, which are basically smelly lizardmen), or xills (four-armed and dangerous, red-skinned people who lay eggs in people paralysed by their venom), or yuan-ti (humans magically infused with snake-essence [more or less], and their descendants), or abeils (bee-people), or avolakias (avolakian? Anyways, giant necromantic worm-like things [more or less] that can mimic humanoid form, have a poisonous bite and 8 claws, heals about as fast as D&D trolls, and prefer eating undead flesh), or desmodu (big gecko-ciliated bat-people that can't fly but have sheets of skin from wrist to ankle, that tame special kinds of bats for pets and mounts), or feldrakes (elves' dragon friends, for when elves won't dirty their hands), or grells (beaked, flying brains with paralytic tentacles and an insatiable hunger), or jermalaines (little rat-fey about the size of actual rats, with no magical abilities past speaking with normal rats), or meenlocks (tiny, clawed aberrations that transform other humanoids into meenlocks), or myconids (mushroom men that grow from cat-sized to giant-sized over their lifetime and gain various types of spores from "distress" and "rapport" to "hallucination" and "animation" [of undead]), or neogi (spider-like aberrations with snakey necks that have umber hulk minions and magically-bound slaves), or spellweavers (weird, six-armed humanoids with great magical skill), or thri-kreen (four-armed mantis warriors), or armands (armadillo-people), or nycters (small, winged batfolk), or shifters (part-lycans). Sorry about the wall of text, I just decided that a bunch of D&D critters would be cool to see in DF, especially since some would lead to the ability to mod in cooler stuff.
But...holy Armok, that's a big wall of text! Again, sorry.
I'll add a bunch of my TV Tropes links later, don't worry. Expect lots of links to "Out Monsters are Different." Maybe bold the monster names or something, to make them stand out.