Environmental Type
In the most recent journal about game development, (and perhaps in the beta ... which I had beta :/) we see that not only does it cost essense to build a new city in wasteland, but ALSO in any terrain type that is not home environment.
Therefore, among humans, I think it would be fun to have a plains race, a snow race, a desert race, a woods race, a mountainous/hilly race, and perhaps a coastal race (depending on emphasis of ocean in-game).
At the very least (for 12 factions, meaning 6 factions or "races" of humans) having 2 desert races, 2 "normal" races, and 2 snowy races. Although having a jungle race, a forest race, a mountain race, combined with a plains race, a desert race, and a snowy race ... might make for better diversity. As far as pigmentation, the snow-race n mountain races would be extremely pale, the plains and forest races would be tan, and the jungle and desert races would be dark or so. (probably desert race to be the most pigmented)
and then for the Fallen, the various fallen races could, instead (or in addition to) of pigmentation differences, the fallen could have actual mutational differences, like ice-cycle spikes for a snowy race, slender physique and vines growing for a forest race, leafy and venomous for a jungle race, sandy/burrowers for a desert race ... night vision& thicker claws for climbing for a mountain race, and where the forest race was slender towards dexterity, the plains race would be slender towards speed, with taloned birds feet, and just over-all better at running over long distances (in their native terrain, the plains).
various factions could also have different magical affinities
-snow are better at Ice/water, mountains are better at Wind/electricity, plains better at life/death, forest better at earth, desert best at fire, and maybe jungle could have some innate magical resistance and disease resistance, even though not as proficient at a single magic type as the others. This could perhaps only really apply to the Sovereign, although would also inherently apply to anyone the Sovereign imbued essence into.
The main reason to have terrain types, I suppose, is even after life has spread and re-awoken all of the world, it will still cost some essence to build a city outside of your chosen environment. A mountain-eer civ, for instance, would have 2 essence to build a city in Ice, 3 essense to build upon the plains/forests, and 5 essense to build upon a desert/jungle.
Meanwhile Desert would be 2 in jungle, 3 in plains/forest, and 5 in ice/mountains. For simplicity sake, forest/plains civs would have 0 essence cost in plains/forest, and 5 essence for either ice/mountains or desert/jungles.