I doubt races will be non organic, as that will mess things up even more than they're already going to be. What would a fire based race use for sustenance, and how does farming/livestock come into play? This sounds like enhancements that can be added after 1.0...
As for extra dimensional creatures (Titans/FBs/Clowns currently) I believe divine materials, slade, and candy are already off the list.
They... wouldn't use anything, like goblins do now.
Goblins raise trolls and shear them for their wool to make clothes.
Would be a bit of a waste if random creature civs all ended up as titan-like critters. No need for food, farming, or anything. Utterly boring worldsim.
I like the part in the PC Gamer interview where Toady muses on how one simple innate ability (teleportation) should end up effecting almost everything we know about society. But slade hulks as actual civs would be a step too far, I think (but modders, go crazy, sure).
I thought you meant food. A fire civ would, for example, coat their weapons with fire to make them ignite people. Sorta like the draconians in my mod, but their armor ignites non-draconians trying to wear it, by the virtue of being dragonfire-hot.