An idea I've had for a long time, but can't seem to ever getting around to doing (time, skill, what have you) is to hack General Chaos in order to make its soldiers appear as 16-bit versions of Team Fortress 2 characters (where possible). This is basically for personal reasons, since I consider that title a kind of spiritual predecessor to Valve's game (the link would be much more noticeable had Valve gone ahead with their idea of adding a general system in the game, as they had once planned).
But I'm not even sure if all of GC's characters are editable that way, so, yeah, that's it for me.