Out of curiosity, is this setting intended to be system-neutral, or is there a lean towards a particular system or type of system?
Right now there's nothing very specifically D&D other than the elemental planar rifts (Air, Earth, Fire, Metal, Water, and Wood) and even
those are only nebulously D&D. The city itself might not fit very well in something like Shadowrun or Call of Cthulhu but probably wouldn't have issues fitting in with GURPS or such. If you mean more D&D worlds, this is very much supposed to be its
own world. I'm not sure it'd fit very well on any other world other than, maybe, Eberron or Mystara.
Also, not a ton of specific demographic information, like races and cultures...is that intentional to allow the city to be system-neutral? Or just haven't gotten there yet?
It's mostly stuff that still needs filled in eventually. The Standard Races/Species are definitely present and, here, include Kobolds, Goblins, Hobgoblins, Bugbears, and Orcs (I think these are included more often lately, but I want to be explicit about it). Ellindam is the type of city where very nearly any sort of race can be found just because it's very large and very influential. Much like London and New York, there has been a
lot of immigration over the years.
A couple of things floating in my head on this (and a bit beyond the city proper):
1) The Fey are responsible for a lot of weird things. One of them is possibly changing Humans ala
Changling: The Lost. If they aren't directly responsible for
all the races/species there are certainly ones that
used to be Human and were changed by them. This is also why a lot of the Humanoids can successfully interbreed even though RL genetics would not likely allow such progeny.
2) I'm currently uncertain as to the actual cosmology just yet beyond the elemental rifts and that there are real deities or beings powerful enough to pass for such to merit a temple being built for them.
One recurring thought is that the Fey play quite a bit larger roll than usual, bossibly the more powerful ones posing as deities (Disney's
Gargoyles being inspiration for that).
Undead can, in fact, simply be Fey (
Banshees and
Dullahans being cases in point). Other types of spirits like
daimons and even souls exist to explain more elemental-type beings or even talking animals akin to the animal yokai from Japan. There's a pretty good
Tumbler on how the lines between Fey and Undead can be very blurry indeed.
Given the above, if there
are anything like Aasimar and Tiefling they could be descended from Fey (Seelie and Unseelie, respectively) or been experiments to "improve the looks" of Humans (Fairest and Darkling
Seemings, respectively).
3) The Goblins have
very malleable DNA (or whatever passes for it). That said, it's a bit harder for such traits to remain consistently over the course of generations (this is also partly inspired by the
Gremlin films). Hobgoblins and Bugbears are two notable exceptions, though
why is unclear. Kobolds might also be subject to some interesting variations too, though I like to think they have better luck retaining features.