How to decide what races to use for a setting?

[Xeviat]

For my setting, I wanted a mixture of classic mythological and legendary creatures. Due to my obsessive love of symmetry, I needed to make sure I had a race for each of the four elements, I needed one race from each world, and a couple more to round things out. I ended up with 10 base races.

What I hope will make my setting unique is that I am approaching the races from the angle of science fiction, determining how they evolved and such in order to make them feel more organic. Since my setting is going to be used for games as well as novels, I've tried to make sure that certain features are only held by two races (I have two strong races, 2 stealthy races ...), that way there is variety amongst player races.

I believe your choice of races is dictated by the feeling you want to portray.

[SilvercatMoonpaw]

1) Have other civilizations of comparatively less prejudiced natures come into contact with the hypothetical civilzation described above.  Conflict (not necessarily war) results, almost inevitably.

2) Have unfathomable intelligences be the baddies, like the Borg, or the aliens from the Alien franchise, or the bugs from Starship Troopers.  These make the perfect "endless Nazi" bad-guys in a pulp-style game since they usually aren't fully fledged-individuals as typically defined but glorified organic machines (to steal your term).  They don't have families or friends or feel human love, empathy, emotion, etc, even if they're not mindless per se. I could see why you'd potentially want to avoid this option (given your distaste for unfathomable intelligences), but I think when we were talking about "alien minds" before were were talking in terms of playable races, or races within a central civilization, rather than villains.  I think you also imagined a sort of Evil Emperor villain in one iteration of your sci-fi setting, with endless henchmen; this amounts to the same thing, really.

3) Run an exploration-based game set in the abandoned planets of defunct civilizations.  Combat would be with dangerous wild animals, extremely primitive tribal creatures, or mechanical guardians.

4) Run a very low-conflict game as typically defined, more like a drama/soap opera/campus novel than a typical DnD game.  The game would revolve around personal relationships, romance, subtle political struggles, and the intricacies of life in the utopian super-civilization.

Or, of course, mix between all 4.

I don't know whether this counts as a mix, but you forgot one:

5) Keeping the status quo/fight the forces of chaos.  This is how you define games where the setting is mostly good and orderly and the PCs play agent who have to fight the random elements that crop up who haven't gotten the message.  This is the sort of game where you play good cops, secret agents working for peace, paranormal investigators, and Silver Age superheroes.  The point is that the conflict isn't with one entity and probably not for long at any one time.

[Steerpike]

Yeah, you're right; I'm surprised I hadn't considered that.  A lot of my adventure outilnes for the Cadaverous Earth, oddly enough, function like this, in a slightly twisted way; there's a status quo (though it isn't good) that's fairly immovable, and small threats crop up towards it, most of them things beyond the pall even by the standards of the Twilight Cities.