As a kid I loved Choplifter on the Sega Master System. I'd imagine all sorts of scenarios to make it more fun.
Combine that game with Starcraft/Command and Conquer. A side scrolling 2D RTS game.
Two sides, a river in the middle. Build up your base (which would extend underground as well as horizontally) and build Helicopters, jets, missles/nukes, tanks, boats (which you can manually take control of at any time).
Goal is to take out the "Leader" of the other side. If you've played Choplifter you know the "people" were tiny little squishy things that you could kill by just landing on. Your workers and soldiers and Leader are all extremely vulnerable squishy guys. Of course defensively you'd position your "leader" as deep underground as possible, or in a building surrounded by anti-aircraft missiles, etc.
Mainly a 2 player, 1 vs1 game, but more than 2 players possible if the world wrapped around.