Diplomacy and Ally ...

[Cohen]

Should Dom2 have an improved diplomacy?
Somewhat like allied status that can allow to pass over allied territory or have common armies (well, the sum of X's Army and Y's Army in the same province) ...

Perhaps extabilishing more diplomacy status (Ally 1st level, share map, Ally 2nd Level, allow allies to pass on your land, and so on).

[Gandalf Parker]

Its been discussed many times. And since the early days of Dom1 diplomacy has been increased but its not high on the devs list.

Most of what you want is possible in a multiplayer game though it takes more effort than just turning on an alliance.