I came up with an idea long ago, which was touched on in here a bit. Basically, after the soulstone is shattered, both this world and the plain where the demons come from are not whole. To fix that the heros must take the battle to their door. Imagine fighting Mephisto, Diablo, and Baal....... at the same time. Final showdown to save the world.
This would deffinitlly have to be a darker game then Diablo 2. I picture dark red sky, posionous fog, and a $h!tL0ad of minions. Possiblly take the characters that you have now and just expand into this new game. Only problem is runewords dont work, and every item you wear is now cursed. Only way to survive, bring it back to the basics, and get new gear. Of course the gear will be cursed in this demention, but that will a positive effect, in this backwards world.