Well, if it were possible to do, I'd say fix multi-player. That includes the various forms, such as hotseat, &PBEM. Playing against a human would overcome the A.I. deficiencies, and while not exactly what I consider civ, it would be an interesting & challenging war strategy game, and well worth the cost. A sort of Panzer General for all eras.
As for Single Player, I think my priority remains diplomacy. Yeah, I have modern era crashes on huge maps, but I just start a new game... it's not as if there are victory movies yet, are there?