A) I dislike the generic fantasy, Tolkin-ish races. Elemental has nothing to do with them, and this is good in my opinion.
B.) This is still a fantasy world though, and there's no reson why we won't see non-human intelligent beings.
C) The entities I describe here are *not* factions, but rather elements that factions can interact with.
Now that the introduction is over, let's start with the bestiary!
Non-humanoid species #1: The Empire of the Ants!
Imagine an ant hill. Now imagine the hive, the 'whole', as an intelligent being. Also, just for kicks, imagine the ants as being 10 cm in size (or 1m if you wanna go nuts) instead of a few milimeters (non-metric users can go *censured*, because I don't care). What this gives you?
Ant hills the size of towns and cities, sprawled over, under and through hills and mountains. Empires, magnificent and majestic, that develop alongside the other factions.
These 'empires' are not the same kind of factions as the 'normal' kinds. For one, you are considered in conflict with them until you discover a way to communicate with them. Not that they are necessarily bad/evi/aggressive/[enter misconception]. Anyway, those ants can be a very important force in the region they inhabit, and being able to communicate with them could be a valuable tool.
It's hard for me to put my vision into words in English, so I'll leave it as is. Take the concept (huge, sentient ants), and see where you can get with it. Also, please note that I didn't use the word 'Zerg' and I prefer you guys not to as well.
Non-humanoid species #2: Elemental Spirits (nothing to do with the game's name)
The spirit of a peaceful spring, the spirit of a raging volcano. The spirit of an ancient forest and the spirit of a blazing desert. These are all real* beings, that can be interacted with. Mainly through summoning, or 'calling' to be exact, a spirit of a certain area/thing can be asked to manifest itself. The manifestation would change according to the nature of the spirit of course, so the forest spirit would be a treant and the mountain spirit will be a rock elemental.
Now, you ask, after we called to it and it came, what shall we do with it? First of all, I suggest you be nice to it, since you don't want to anger the river that pass through your town. Now, maybe you want to ask it to change its course so it would pass next to your town instead, and free some precious building slots? Or maybe ask it to create another stream so that your town won't get flooded at the winter? The possibilities are endless!
I gave a more freeform example, I'm just a bit tired at the moment and can't be arsed to think of a strategic example. This idea was originaly developed for a pen & paper RP system I'm working on, and even though I don't even hope for it to be implemented in Elemental, I thought it might interest some of you.
OK, now you try it! Think of a sentient species that can't possibly be a standard faction. Mermen live in water, but use the exat same rules that human do, for example, so they are bad. Dragons, on the other hand, have potential. I'll write about them tommorow if no one else will take the challenge.