If there's one thing we know about the Prime Evils, it's that they are constantly scheming against each other for supreme dominance. I wouldn't be shocked to learn that such a conflict is currently raging within Prime Evil Diablo, so how about a fight where during each phase one of them assumes "control" of Prime Evil Diablo's body/appearance, allowing us to fight all the Prime Evils in one epic battle?
With Diablo being the last form... Yeah that would be great ! And killing them is re releasing them all.
But i REALLY fear that the next expansion will be on a diablo / imperius merge or something BAD like that. (we fought demons then angel then angel and demon mixed ?)