It would be awesome for replayability and just fun in general if there was a checkbox before starting a Co-op game that allowed for 99% shared access to units, buildings and resources with the exception of the player hero.
What I mean is a single resource pool, shared between both players. And the ability for each player to simultaneously control any units on the field. What this would allow is for players to become leaders of their own playstyle. For example, I could be the city builder, and economic manager, placing buildings and getting out finances under control. While my partner could do what he loves best, managing the army and defending our great city in the desert.
Please give this a good think. As it is now, you really have to play as 2 solo players early - mid game.