I know this might sound like a dumb idea, and I also know I already have a mod in progress that's never actually been played, but I'm thinking of doing a MOO2 mod - trying to recreate as much as possible of MOO2 within SE4! I've come up with equivalents for most of the techs, now to code it and see if it balances...
If you want to see my ideas, they're on the new SE4 section of my webpage, which is
www.geocities.com/edwardkolis