Yeah, I was actually considering integrating something like that into X@COM, though that would certainly be much later, and it would essentially be a different game. A couple of ideas I came up with:
-It's a little ways into the war, more and more nations are turning to the alien side, and you play a lone soldier disgruntled with X-COM's ineffectiveness. You strike out on your own and play a more-or-less one-man roguelike adventure across earth as you find new tech, meet NPCs, and eventually find a way to get yourself to Cydonia itself and stop the alien menace. More overworld-oriented, except for bases which could be similar to dungeons. Would be an entirely different experience, but definitely fun
-Much more closely integrated with the original: As commander you are an actual member of X-COM and if your location is discovered, you have to fight off the aliens in a different roguelike mode with soldiers assisting you. Easier to do, but still a separate game.
I had some others I can't recall. I'll stick with the original for now, though, because it's going to be pretty spiffy once I get the real interface running (haven't even started on that
).