We'll think about the faction composition later; first, we need to decide which factions will be included.
Here are the factions I suggest adding to the game:
Atlanteans (from the neighboring thread)
Necropolis (a mix of elements from the third and fifth games)
Forge (not fully developed, but the idea is that the Bes people will live there)
Humans (I envisioned humans as Slavs)
Mages (Turks)
Underground Lords (from the third game)
Elves (standard)
Dwarves (Scandinavians)
Lizards (in the neighboring thread about the ninth race)

I've removed the less serious races.

Added 45 minutes later
I'm honestly not sure whether to include an aerial level in the game or not.