I bet my left ....hand, that Diablo is NOT in Diablo 3 original release....
Anyone wanna bet me?
Here is why I figure so: *warning massive amounts of speculation and fun ahead*
Azmodan has already been pegged as the BAMF of this game.
He is our main enemy, it is a race against time to beat him.
That being said, Belial is another boss, along with the Butcher supposedly. Most people know this.
But here is the thing: in D2, it was quite LACKLUSTER to go from Diablo, the biggest BAMF of all time...to Baal...the equivalent of a Demon in a wheel chair...with a clone....for some reason.
BAAL was no where NEAR as FUN as Diablo was.
Because of this I wager: That in the final cut-scene of Diablo 3, we will get a lead on to the expansion (like the epilogue in D2).
This cut-scene will reveal some kind of horrible epiphany, that we were fools all along, and the real threat was elsewhere.
I would wager something along the lines of Diablo making a pact with Imperius, where they Intend to destroy all of Sanctuary once and for all. Eliminating the Nephalem as a threat, and resetting the conflict to JUST Angels VS Demons.
Tyrael caught wind, and was backstabbed. Imperius lied to the Angris Council, and Tyrael is banished from the Heavens...
So not only Will Tyrael have a bigger role in this game, but the final fight will be between us and Tyrael vs Imperius...
Leaving Diablo for the expansion, Diablo 3: Terror Reigns
Well...that is what I am speculating anyway.
At the end of the day, I think Diablo will be saved for an expansion, as to build further tension, and END the game off on a high note...BUT, Diablo fooled Imperius, as he had a more sinister motive all along...
IN expansion 2 to Diablo 3, we will face an even bigger threat.
After Defeating Diablo for a 3rd time (man he sucks at this) we find out, that in banishing ALL prime and lesser evils, that they returned to their ORIGINAL cosmic form.
Tathamet! In the second expansion, The Nephalem, along with the High Heavens (which now has Tyrael as the head of the Angris Council) will wage all out war on Tathamet...
This is what Cain (who dies in D3) was trying to warn us about, NOT Azmodan, or Diablo, but of Tathamet returning, to end all things, all life, and live in eternal bliss, knowing that he finally BEAT Anu, and won a long waited victory, set in motion billions of years ago....ooooh, I am getting all tingly with excitement.
All just good fun and Speculation, but I think something Like this will play out in Diablo 3, and if not, well I had fun writing this.
*END OF MASSIVE SPECULATION*....enjoy!