[Cohen]

I believe instead the Diplomacy should be implemented especially for MP games.

With various level.

No level means, war or indifference. You see the map as it is now.
A level of non aggression treaty-peace allows you to see with precision what's in the bordering provinces of the enemy territory.
A low alliance setting that allow you to share map vision of the banners, or perhaps to share the banner+dominion vision.
A fully alliance set that shares military vision too of your troops and the one of the allied player.