Surprised this hasn't been mentioned before (at least Search didn't turn up much), but I think the Half Life universe would make for a great Xcom mod.
Mainly, I would just like to see the aliens from HL replace the stuff from Xcom.
Maybe do something with an alternate version of the HL timeline, with the NATO forces or Xcom or whomever trying to figure out what's going on. Start off with the standard real world weapon, and then with intense research, have the ability to research the gauss and plasma guns.
I remember reading a thing somewhere on how to add aliens into OpenXcom, but I can't seem to find it any more. If anyone wants to run with this idea, I'd be willing to do sprites
Or if someone can point me towards some tutorials on creating content for OpenXcom, that would be appreciated too.