Crazy thought: I think Diablo is going to have to pick up more MMO-style features in order to keep replayability high. But I don't see it becoming a WoW clone. If anything, I'd imagine a World of Diablo to be much more akin to Guild Wars. That said, I'm not super crazy on the idea of a GW clone. I got bored with GWs way faster than I ever did with WoW. But maybe that's me.
Just my $0.02.