Diablo III seemed like the sort of game that would lead naturally into an MMO for the franchise. The online functionality is there, the options are there, and it's not as if Blizzard's big MMO didn't derive quite a bit from the framework of Diablo II, after all. But according to content designer Kevin Martens, there are no current plans to take the franchise into an MMO space, even though there were plans to do exactly that not too long ago.
The original thinking was that Diablo III would be the sort of game that could evolve its setting to a point where an MMO felt organic, natural, and appropriate. However, a shift in corporate culture and overall development focus cut those plans short before they went much further than discussion. Seeing how players enjoy Diablo III and what they like about the game has resulted in a shift in focus with more future content for the game... but no MMO exploration.