I like many of the comments here... but with Gandalph, I
really agree.
Especially about falling back one level.
And Slynky, I think you are on to something with the league rankings... but I think many would argue the fact that your empire score is irrevelant in single player rankings/matches.
It would be great to devise a system based on current rank and the rank of your opponent, and points awarded for victory.
www.spring1942.com did this very well for an Axis and Allies league. Not saying that this would be easy, but I could refresh my memory on the calculation they used, and check back in if you are interested...