We have dozens of heroes still alive.
Conflux. We clearly have a leader for the Conflux (and since the Conflux is subordinate to mages), that's Zihir. If you want to make the elementals the bad guys, okay, let the Nth necromancer raise a rebellion against the mages.
Empire. As we already know, there won't be a new Empire; someone will conquer it as well. Frida and Duncan are still alive; they weren't taken out in one fell swoop, and I don't believe Duncan will just sit around and wait for someone to knock on his door.
Dwarves. I don't know anything about the dwarves; that's where we need heroes. And whose army will Wolfsten come with?
Magic Chimera. If they're at war with the knights, then things will be tough without the Empire.
Orcs. There's a huge, unified orc army under the command of Gotai, but as far as I know, this is about other orcs from the Archipelago.
Sanctuary. There are no heroes here; maybe Fae'daena could send some here? (And two of Irollan's generals are still alive.)
Necropolis. Anything could happen here.
Lizards. Similar to the campaign about Gotai.
Demons. Let there be all sorts of Sarahs and others; they'll attack someone.
Free Cities. Railag and Tieru.
As you can see, we already have many unresolved heroes from the classic fifth game; where else do you want to cram in the Kirillovs?