Star Wars Vs. Star Trek
And I know, I know, we all know who would really win, but for the sake of fun, put in the Star Trek ships and either make it "realistic" and have the Empire go on missions conquering the Star Trek galaxy, or fudge it and make them both about equal in an all out epic war to the finish, with all the Trek powers (Federation, Klingons, Romulans, Cardassians, Bajorans, Dominion, Hirogen, Borg, etc.) united on one side vs. the Empire and any other cool units on the other side.
Imagine an expeditionary Imperial Force going through a wormhole to the milky way galaxy of the 24th century. Resources are scarce at first with reinforcements not yet able to get through until they've setup a base, and go from there.
Granted, this would be a big undertaking, but the easy part is that the Trek side doesn't have much in the way of (canon) ground forces. The real task would be doing the fleets and things (mostly capital ships, but some fighters).
Similar mods like this have been done for other games I'm told, but Empire at War it would really be cool, because of the real time ground AND space battles all in one game.
Feel free to use my idea if it strikes your fancy, modders!
Edit: We have a thread about it now, here