IMO for alliance we need to try as much as possible to actually make it official in game. An idea I thought of for allies is say:

Carthage gives me 5000 dennari. AI Greece is happy.

I then give money back to Cathage and Carthage AI is happy.

Carthage never loses any money, but then the ai is more likely to like each other. (also, it'd be excellent to be in game allies so you can have joint sieges)