Kind of a ladder model? The bigger you get the better they get?
Im not sure if farm upgrades would neccecarily be realistic, but maybe higher moral units to represent them trying to halt the mighty empire.
A lot could be done if there was a more advanced diplomacy and trade model. Rather than the AI just wholesale being aggressive they would begin to cut you out of the political/economioc loop.
But onto the main point, the last few campaighns, Ive been extending the game by basically idulging in little diversions, razing even well built up provinces that I capture when I have a blood thirsty king, spawning nothing but bishops to send out on conversion campaighns if my king is pious, that kind of thing. A campaighn or so ago, me and my brother played "a king at a time", where we'd try and make each kingshiop distinctive from the last which slowed down the Empires growth substantially.
Just little things like that, that hold you back articficially.