It is up for debate, but Eyonix replied a fan at the official forums who stated Blizzard is done with Diablo franchise.

Eyonix: This reminds me of the time that I read, “Blizzard Done with Starcraft.  It looks that way don’t it.”  You never know what the future holds, friend.

All Starcraft fans thought the same for Starcraft 2, when in fact the last 4 years Starcraft 2 has been in development.  Basically, development of the sequel started 4 years after Brood War expansion hit the shelves.

A Diablo game is coming.