How about making it so you actually have very little true expansion? Like in the actual Roman Empire after a war is complete you gain direct control over the closest provinces and then the nation that you beat can then become a client kingdom which pays tribute to you? If that was possible it would really spice up the peace time through the ability to fine tune the breaking points of provinces taxation and you would also have military responibilities towards your client kingdoms. One thing I would love to see implented is a use of the Crusade perk to allow allies to actually have a diret affect on a war by sending a large stack of troops to a destination of your choosing. Like lending or supplying local troops for the Empire (which actually happened) and then having a decent amount of control over them. (ie destination) This could also possibly be implemented for allies? Would make diplomacy much more interesting.