I didn't want to put this in the suggestion area, as I feel that's for more 'integrative' material in the current game, but I had an idea for a mod. Suppose that instead of playing humans and defending the earth, you played as the aliens and had to invade? The geoscape would be the same visually, but you'd be able to select craft to terrorize different cities. There could be special missions for smaller craft to obtain resources and research different areas - thus allowing you to build 'alien colonies' only after you've studied an area for a time. Your starting base would be larger than a standard XCom base, but maybe additional colonies would be more costly. The battlescape would be similar as well, but suppose that instead of a fight for every terror attack or landed UFO mission, there would be a chance XCom wouldn't show up (maybe a small one, but we've all skipped a landing in the middle of siberia when we're occupied with the terrorist attack in Cairo). Maybe for UFO missions a handful of units start the battle in the craft, but the rest (and for all terrorist/base missions) your units would be pseudo-randomly placed around the map (ie: it is more likely you'd have units in or close to your own craft, but farther away from the XCom craft).
Some things would have to be completely redone (economy, research, mind control, etc...), but it wouldn't need many other things (like graphics) other than what the game already comes with (except for maybe background images - we'd need more of them)
