Honestly,
If they turned Cain possessed like they did Gris...
That would be lame, except if he was there to do some 'sriusbusinuz' (Lol)
Like floating in the air, surrounded with a shield of thunder, summoning old Diablo I monster's
Using his all knowing power to cast some sickeningly amazing spell's
But im pretty sure he wouldnt be the 'Final' Boss, even if Cain were turned demonic/evil/possessed - Etc.
As for Diablo, they COULD possibly make him the 'Final' Boss,
And then when they release and EXP to Diablo III
It could be Tyrael, 'corrupted' as many of you are saying could be true
Either way, I want Diablo to be a TANK of a BOSS
Also would LOVE for them to release a Tyrael Set
Alway's HOPED for them to release one on Diablo II
For the Paladin job class, of course
But now it seem's there isnt a Paladin in Diablo III..
But would still love to see a Tyrael Set or something along those line's