My colleague and I (fktifzobr@mail.ru) discussed the Factions and agreed that we should make Haven not a purely human race, but a mix of dwarves, elves, and humans.

Added 2 minutes ago
Here, I found something:

The Overlord is vulnerable only to Railag (remember the Heroes 6 campaign), so Sareth probably won't be able to overthrow the Overlord (he clearly doesn't have enough strength), so we can't say anything specific about the demons. If Sareth freed his father, then he could have done so on certain conditions, but it's unlikely that the Overlord would listen to him (well, this is just my opinion, but it seems very obvious to me).
But the guess about the Necromancers is interesting - how can the Ring have its own inner dimension, as in Heroes 5, if it was destroyed in Heroes 6? I think there's some truth in what you're saying.
It will be difficult to keep everything within the lore, and if we believe that unverified information, then we currently have three sides: the Order of Void, the demons, and everyone else. By the way, frankly, I find it hard to believe that we can combine dwarves and darkness within the lore - dwarves would rather go underwater to Shalassa than hang out with the Faceless Ones.