I've figured out who lives where, who has what kind of magic, and even have a basic plot, and know generally who some of the major and minor characters are.
I've worked out that there have been a couple large population shifts on the continent of interest; the biggest being a massive die-off some thousands of years ago that killed nearly the entire human population on the continent, except for a small group who were chosen by the gods for a specific task. After that, a few different groups of humans moved in from various other areas, though they've had long enough to regard the land they live on as "theirs" (think Saxons in England).
After thinking about it a bit, I decided that I don't want to go with trying to build entire languages for each culture, even if I'm only going to have names of people and places. For humans, at least, I'm going to mostly borrow from Earth cultures, to give that certain amount of medieval europe feel that's such a part of standard fantasy settings. My hope is that having the humans have earthish names will make the distinction between them and the non-human species with made-up names more pronounced - I want my aliens aliens and my humans human.
I also thought about the fact that most of the people would have come in different waves from elsewhere, and thought, hey, there's absolutely no reason why they'd all be white people. I was, in fact, tempted to just have all the white people get killed off in the long ago cataclysm, but I wanted my most European-feeling area to look European. Also, I want to have a small group of post-cataclysm pepole living in one nation as Chosen of the gods, and having a nation of magic white people seemed a little oogy. So I ended up deciding to make them magic black jews instead. Which is to say, they have dark brown skin, vaguely hebrew sounding names, and have special relationships with certain gods. I'm not sure there's a politically non-volatile way to do this, particularly as SuperPrivilegedBoy. We'll see how big a mess I end up making.
I am concerned that all I'm doing is using skin colour as another way to flag characters as "exotic" or "interesting" - the same sort of criticism has been elvelled against the Matrix movies, for example.
My Romans look like Arabs, my Welsh look Desi, and my crazy horned cat people are sort of mocha toned. I want the elves to be Tolkien-y and boring, so I'm leaving them as default northern european (I guess I did end up having my magic white people after all). Dwarves come in all shades of brown. I haven't figured out what to do with the short magic singing people - I want those guys really weird, but green turns 'em into goblins, and blue turns 'em into smurfs. And no one can take a purple person seriously. Grey, maybe? Meh. What can one do with 3 foot tall guys with 12 octave vocal ranges? Or are they destined to be ridiculous unless I make them taller, at a minimum?
Am I overthinking this? Underthinking it?