Here’s a suggestion I have for the game, though I know it may be to late to make this suggestion.
Especially with every player having their own set of units, What if you could tribute units, to aid your ally’s weak-spots in their army? this could help in diplomacy games, for example if an ally was giving you recourses but wants to make sure you don’t use it against them, you can give them some troops as a mutual trade.
Another idea i have would be to pool or “tribute” technologies, basically, investing in a sort of envoy unit that can share the benefit of a certain tech with a certain ally, and dies when it reaches your ally.
Another thing about envoys though, it could be a nice way to simulate diplomatic interactions in the middle ages, although they may be too complex.