Behold! More demons to kill and loot to grab!
Diablo III's inevitable expansion is already in the works, claimed Blizzard President Mike Morhaime when speaking at Blizzard's latest investor call.
We do have a plan, an expansion plan for Diablo
A plan... a plan is good.
I don’t have any timeline to talk about. I think that the most important thing for us always comes down to the quality of the expansion, of the gameplay, and so that will be a big factor in terms of driving our schedule.
For comparison's sake, Diablo II: Lord of Destruction released almost exactly a year after vanilla. However, if Blizzard were to try to pull that off with Diablo III they would be in for a world of hurt.
This expansion might not be coming for a while, which is hopefully the case given Diablo III's depressing endgame state, but I think I speak on behalf of nearly everyone when I say we'd rather hear about future patches and free content. The Real Money Auction House has generated a luxury of revenue for Blizzard and should be more than enough to drive improvements without the need for a $40+ expansion pack.
Many players quit Diablo III after experiencing what was a compelling four chapter journey a few times—including me—but have since forgotten about it. I don't see many people investing money in additional content until the foundation is richer, and hopefully Blizzard understands that.