1. Humans
2. Undead
3. Demons
4. Orcs
5. Mages
6. Sylvan Elves
7. Dark Elves
8. Dwarves
9. Nagas.
In short, something like this, I suppose, might work.

Yes, I think that such a set of factions would satisfy everyone, including me :). It's a pity that in "Heroes 6" they will only include five of them (or maybe they will come up with someone else) - that's really too few :(.

And the desert is an argument :(. Where will the mages live then?
Although, maybe before (the events of Heroes 6), they didn't live in deserts, but in jungles... Or the jungles in the Silver Cities turned into a desert as a result of the war between the mages, the undead, and the orcs... Oh, the guessing game starts again. But who else could be from the old factions? Maybe there will be something like the Rampart from "Heroes 3" - elves and dwarves in one faction? They could hide from someone in the jungles :).