Ahh, there's no substitute for UFO. Believe me, I've tried to find one! There was a WW2 one recently, which is about as close as anyone has come. The thing that made UFO special was the mayhem of a 100% destructable landscape. That WW2 one (sorry, can't remember the title) was good in that everything was destructable, you could shoot through walls depending on the calibre of your gun etc. but it's still not UFO.
UFOs was great because of research, base building (and the fact you could be attacked in your own base), mutliple alien races, mayhem of weapons, and the stats of soldiers. But above all, 100% destructable terrain.
I'd definitely play a turn based sci-fi game if you made one, no matter what. Turn based combat is definitely my cup of tea.
Insiiiiiiiiiiiiiiiiiiiiiiiide!