Here's some more ideas after fun online-reading about x-com (especially X-Com:Genesis)
I mainly want to spice up the old monotonous gameplay.
1) To break the monotony of the game. Could we copy TFTD and have mixed races in some missions/ufos? It's easy to implement and GREATLY improves gameplay.
2) "Terra-forming" missions. When you lose the game, it says the aliens has polluted and mutated the planet, etc. etc. Maybe there could be a terra-forming mission type for aliens? It'd be remote of course (so they could do it in secret) and it'd be grassland/jungle areas (where plants could flourish).
And of course there'd be a mission for us to stop/undo a terra-formed area on earth. The tactical map would be a mix of grassland/jungle with alien garden stuff. Actually borrowing this mission from X-Com Genesis.
3) Alien capture missions. Sorry if somebody has suggested this. You'd need to capture a specific alien/rank to help research or to find an alien base or find out what region an alien base is located (or it could just be words and have no real effect in game). Killing that particular alien would result in failure of course. And lifting off with its stunned corpse would be a success.