Hi!
First off, I'd like to say I've been watching this game for several years now, and have to say it is coming along wonderfully and UFO:AI is an awesome game!
I noticed the "Proposals" page in the wiki and thought I'd start one of my own. My proposal is the addition of TFTD-esque gameplay on top of the existing terrestrial missions. I'll list the things I had in mind:
1) The player can build sea platforms as bases. These platforms are identical to regular bases, except that instead of hangar bays the player can build submarine bays.
2) Playesr can purchase submarines.
3) Players can use the submarines to reach UFOs that crash into the sea.
4) Underwater battles may occur.
5) Aliens may build underwater bases.
6) Additional alien species exist that thrive underwater.
7) Troops/aliens can have neutral buoyancy and swim up and down as well as laterally.
etc.
Not sure if this has been suggested before, but I didn't see it listed on the wiki page.
-Mike