What would need to be changed? Its only a matter of how turns work really.
I'm afraid that most of the game in true Co-op would be: one player playing, the other player watching. One player doing a bunch of battles, and the other is on a dispatch for months. The first player is taking a lot of real time, while the second player is just waiting around. If both players are forced to do the same thing at the same time, it is not really multiplayer.
It would be like an extreme version of playing Civ5 with simultaneous turns. Where one person has a huge army to move, and the other person just needs to click next turn and then wait.
= not very fun most of the time.
I would be fine with seeing multiplayer someday, but something closer to what Chris suggested instead of a real co-op.