Since the Norbals mod, I've been waiting for more mods which address the lack of a rainbow coalition in my colony, various strengths and weaknesses even further randomized by genetic makeup. The norbals have a natural damage resistance, but are weak to heat. The antithesis of this could be lizardmen-- cold blooded and incapable of withstanding the cold for too long, walking, talking, reptilian-humanoid bandits and tribesmen fighting alongside humans for the chance to finally get off-world(with some hearty persuasion, that is).
Orcs, slow and range-less, masters of seiging fortified bases via axe, mortar, or fortified catapult (see the GlitterTech mod's tanks in the Orion Corp for example-- just add the ability to shoot over walls)
And lastly.. the goblins- sneaky sneaks, thieves and assassins. These guys can masquerade as civilians of neutral/friendly factions, and once they're in your base-- BOOM
Literally! goblin sappers are suicidal killers and after the trap is sprung, they run their bite sized soon to be flaming corpses into your colonists and turrets, exploding themselves because they are your friends and friends do that.
Hope you like my ideas, mostly stolen from warcraft but it was fun writing about it anyway. This kind of thing is suggested all the time I feel, so instead of serious balance discussions, why not just throw in a few ideas of your own? Do it.