If the developers are still looking for suggestions, adding the ability to share control of a country (assuming that isn't a significant change) would add another dynamic for players who are looking for a challenge.
Part of the fun of cooperative gameplay is beating harsh odds using superior teamwork and skill, and a game like DC which benefits from micromanagement (the dragon and unit abilities) lends itself very well to this idea. There is a lot of satisfaction in beating two AIs with one excellent army, or the hardest AI, and this is one of the reasons to play co-op over being on the same team, as generally the odds are worse and the challenge is higher. There are other possible benefits, like training another player on how to play strategy, helping in RTS mode, and would also satisfy the players looking for dedicated RTS commander and dragon. A further refinement of the idea might be not allowing the dragon controller to despawn for a time after taking damage to encourage more tactical play (perhaps a server setting), making the dragon a more in-depth experience than would be possible in SP.
Finally, sometimes it's just fun to win the game together as one country instead of two. This really seems to be the only RTS-orientated multiplayer option the game is missing. If there is thought of another patch, hopefully this helps!
