i dont really think thats the way Diablo 3 would be played out, only because it seems kinda pointless to make the whole game over then just add 1 more act. and well 2 different classes as well. i think that, if that were to happen it would be a second expansion just like Diablo II: Lord Of Destruction.

If Blizzard were to make Diablo 3 i think they would make a whole new game... meaning Diablo and Diablo II are way different from each other which means Diablo III would prolly be way different than Diablo 2
But if your correct and Blizzard makes Diablo 3 as you say, which thats a rumors lemme remind you. I wouldnt buy it, i wouldnt wanna waist my time doing all of the same shit over for one more act and 2 different characters. Plus you would prolly end up paying 50 to 60 bucks for it... ( Yeah Right No Thanks )