First thing; Tyrael will not be evil (he is one of the coolest character Blizzard has made). I think will not be like Arthas or Kerrigan corruption, Blizzard has much more twists in the story.
Yes, Tyrael will be punished that's why he is casted out from the Sanctuary.
Second; that Hydra-like demon is not Diablo, its Belial (The Lord of Lies). It's his true form with three mouths (which means more lies).
Third; that archangel in D3 website is not Tyrael, its Imperius (Archangel of War). If you happen to see the trailer, there is a comet heading straight to the tall building in the town, its Imperius heading right to get Tyrael (who happens to be in there to consult Cain some important matters about the Sanctuary). And i believe that Tyrael's face is COMPLETELY Invisible, that archangel guy's face in the D3 website is partially visible. One of the cinematics in D2 shows that Tyrael's hood is cloth not metal. The armor is different because in D2 he has glowing gold set and the sexy wings seem to be incomplete.
D3 will be like you and Tyrael fighting side by side against the High Heavens and the Burning Hells.
Diablo WILL return to be a final boss (again). The 'Thousand Pounder' and 'Siegebreaker Assault Beast' will be like level bosses. Lilith will appear too. The place where the worldstone once stayed is where demons will pour out from the Burning Hells. You can see it on the trailer.
Anyway, D3 will be SUPRA COOL (a whole new level of super).
And if you still wanna know about D3 plot or Diablo Lore check this site:
Has Tyrael been working with the Prime Evils from the start? - Diablo 3 & Diablo 2 Forums
I got all these information here.
D3 rock on!