I don't see any reason majorly different factions wouldn't be a good idea. Just look at the differences between Crusader and Arab armies circa 900, or Incan and Japanese armies circa 1400.
I'm not saying "top priority", but you see Elves as a unified race too often (or split into different related races). It'd be neat to have Dark Elves be regular elves who aren't quite so... elegant. Not just dark-skinned angry versions of regular ol' elves, but... well, you get the drift.
I'd give up every Elf everywhere if it meant steam-powered tanks! Steampunk is always welcome!