1) I'd love to be able to play as the remnants, as a "what if" they got the Forge of the Overlord instead?
2) I'd also like to see more lieutenants, so it isn't the same two every game. Maybe some of the other old Elemental Lords?
3) More heroes, baddies, quests, loot. This is a game that can be expanded via the toybox, and needs to be.
4) Some more unit options/resources. Perhaps have some resources that have wildly different uses based on faction (though shards are always the same)
5) Ways to increase the challenge. I suspect folks will be beating the game on insane in 2-3 months regularly. (Though IMO fast doomsday counter+ Hard > regular timer+ Insane)