It would be nice if multiple players could share control of the same civilization in multiplayer. They could split up tasks among each other - one would deal with moving units, another with production... or they could each take care of a region of the civilization, or they could decide to split their responsibilities in another way or not split them at all (the decision would not need to be enforced by the game). It would make games with huge civilizations more managable and also probably speed them up.