If I could change 1 thing about the AI:

I would change the decision process for retreating before battle. Nothing makes the game more of a letdown than to build yourself a great army. Face off against the top contender on the map....and then have them run to their castle without a fight. Often they lose tremendous property and strength in the process. I've destroyed whole powerful nations much bigger than my own piece by piece that way. The biggest threat in single player comes from rebellion and reappearing factions.