The military access could easily simply work in the same way as with crusades. In fact, you could have to create an "Alliance army" that would function like a crusade in that it was only able to move to a target province (eg that of a common enemy).

Improving the AI so that it wouldn't retreat so often, would be good. In particular I'd like it to choose to defend more easily defensible provinces in favour to less defensible provinces, etc. I'm sure there is an algorithm that decides whether the AI retreats or not (based on the relative sizes of the armies) and this could be expanded upon and tweaked.

The main change I would like to see would be to be able to play against other humans in the campaign game, with battles fought against them. This would be some undertaking...