"Diablo3" is already in the works and is going to go beta sometimes "soon". Actually by that I mean Hellgate London...
As you probably know Hellgate London is developed by Flagship Studios(aka the brains behind the Diablo series that left Blizzard and formed their own company). For more info go to:
http://www.hellgateguru.com/
There are plenty of movies, trailers, screenshots and interviews with the developers. This game appears to be Diablo3 with a different name. As far as I'm concerned Blizzard better make SC2 and tap into that goldmine because Flagship Studios has already beaten them in the Diablo race.