Well I'm in agreement, no multiplayer, I'm a sp player never enjoyed mp.
I was thinking more on a Etherlords 2 style campaign,
We've seen the human campaign, I would like to see the dwarven, elven, undead, demon and orc campaigns
In Etherlords 2 they were really lazy, you had these four really massive maps, and four races. Now each race (you could play two and then unlock 2 after all of them play the final uniting campaign) started in their own area and finished on a different map. You would have the starting maps easy, working their way to the final map hard. (Demons start in Demonis, Elves in the elven island) etc.
It an easy idea, no new maps to create, but you would have three new classes per faction, so the elves would have rangers, sentinels and druids, the undead Unholy Champions, Fallen Paladins and Necromancers etc. Change the skill trees slightly, or completely give the undead bonuses to undead, the elves bonuses for a pure elven army etc. You would of course need a whole bunch of new kings quests and side quests, items, maybe some new units to balance the armies out etc.
Only problem would be balance issues, the humans mostly scale well, the dwarves are very mid level, the demons high, leadership costs do start to balance that out.
Good thing about this approach is it's fairly easy to implement ,they could even release the campaigns in a series, running a story along the way.