[Edi]

It'd be easy to do if ally setup would be from the beginning of the game and can't be altered during the game, that would enable teams. It'd cause a lot more problems, though, because you couldn't attack allied provinces or troops, which would be a logistical nightmare if you needed to take troops through allied territory. Unless the allied flag would allow "sneaking" through allied territory so that the ally sees roughly how big an army is moving there but can't do anything else.

The downside would be prevention of the ruthless game of diplomacy and opportunistic backstabbing that MP now entails.

Edi