Yeah, on note of that necromancer idea... maybe not all the high council or whatever sacrificed themselves for that protective ward thingy in act 5...
I know that nathi..eeeak... something or rather didn't sacrifice himself and you had to kill him, but what if maybe another guy also didn't and... he was the guy (evil necromancer).. maybe he got pissed that Tyreal destroyed the World Stone, or got corrupted by baal previously.. i dont know, just thoughts.
Maybe he somehow manages to kill Tyreal, and ressurect him from the dead or something... and ressurect Diablo (like you said). you could even go as far as him ressurecting Tal Rasha and other "not-so-publisized" characters. anyways, once again just more thoughts throw'n out there for you guys to criticise
Anyways, it did mention that there was no set consiquence from destroying the World Stone, so there are lots of possibilities