I know this is not really a new concept, but I've recently begun playing D2 again, and letting some of the story dialogues play out, and watching the cut scenes again (love these, D2 is so good).  Doing this, I am reminded why I decided that Tyrael should be the ultimate Diablo villian, even years before D3 was ever announced.  In fact it almost seems painfully obvious that they were setting him up for this in some cases.  The question is, is it just coincidence or were Blizzard really thinking so far in advance?  That is, did they already have an idea about how the story continues when D2 was made?  Here is the dialogue that I encountered when after kill Duriel last night (courtesy of Diablowiki):
 
Cain:  
"It is highly unusual for the forces of Heaven to so directly interfere with man's destiny, but Tyrael was said to act of his own volition. We have never been able to discern why. Perhaps, he goes against the consensus of Heaven because he doubts our ability to defend ourselves, or perhaps, he sees more threat than his peers. Where the actions of Hell often seem straightforwardly bent on destruction, the motives of Heaven are unfathomable." 
 
So Tyrael's motives for helping men when heaven usually stays out of it are unknown..... no big deal surely.
 
But what got me thinking about it in the first place many years ago was in act V,  with the destruction of the worldstone.
 
Anya:  
"Strange that you say that the Worldstone must be destroyed. The prophecies said nothing about that. Perhaps all we have fought for will be lost--or perhaps we'll never need fight again!" 
 
Malah:
 "If Tyrael says the Worldstone must be destroyed, then it must. We cannot let Baal's corruption prevail! The world will change, true -- but who is to say it isn't for the better?" 
 
Qual-Kehk:
 "The destruction of the Worldstone does not bode well for our world. But I'll try not to worry... After all, we have warriors like you fighting for us and for the Light. Farewell!" 
 
All these citizens of Harrogath seem a little uneasy about the destruction of their sacred stone.  That's all I've got for now.  I'll keep my eyes open as I play through the rest of the game and see what else pops up in favor of this theory.  Also - Tyrael has that awesome moving background on the D3 website.  
 
I'm sure most people agree that Diablo 2 isn't about the story, but rather the loot.  I still think the story is good, and that it's backed up by the absolute best CG cutscenes of its time.