I think this would be a great optional idea for difficulty levels above normal.

A "REVENGE" option given to the AI when the human player wins. The next game he/she plays gives a few more bonuses to the AI and also make the AI MORE AGGRESSIVE to the human player each time he/she wins until they lose, then it resets itself. It would also have the ai's as the human player wins to be more likely the ALLY with itself and thus making it harder and harder for the human player to win. Afterall nobody likes the same player winning all the time that's why in Diplomacy and games with lots of human players you hardly see the same person win twice because of what? REVENGE! lol