The Diablo games have been such a huge part of my gaming life that I'll be excited no matter what they announce and I'm assuming they're going to announce something, there's been too many hints at it. It's going to obviously be one of 3 things: Diablo IV, Diablo III Expansion, or Diablo II HD remaster.
I'm torn between what I'd prefer between a Diablo II HD remaster and Diablo IV. I enjoyed Diablo III after the new team got ahold of it with RoS, but vanilla was extremely rough for a while. I think they realized how bad they butchered the story and atmosphere in Diablo III so I'm hopeful IV would be a huge attempt to get back to their roots. The Diablo II HD remake seems like the most fail proof option of the 3. I reinstalled D2 a couple of months ago and have once again been having a blast with it. I revisit it every year or two and consider it my favorite game of all time. An HD remake of it would suck me in so hard.
Diablo III expansion? Meh, a distant 3rd choice for me. If they find a way to resurrect Cain, that would help its cause though.