I was reading about this stuff just the other day. Here is what I read (and what seems interesting about the possibility of a Starcraft 2):
Apparently rumor has it there will be a 4th race. I saw this video on google, and the extra character in there looked like a protoss zealot, except with some extra arms with claws coming out, which matched up with the storyline of the secret Zeratul mission in which you uncover the protoss-zerg hybrid. Maybe the new race is prozerg or zertoss; or maybe it's something new all together--if, in fact, there is a new race, or a Starcraft 2 for that matter. The site I was reading this on said that there was a leak from Blizzard, and it had something to do with Korea.
As for Diablo 3, I was reading about the MMO possibilities. That there are two sides: Angels and Demons, with the humans being split between each side. If you consider how the game ended, they could easily do an MMO take on the storyline. The prime evils may be destroyed, but without the world stone to keep the forces of hell at bay, there could be a never-ending war between the forces of good and evil for the control of the world. If you have ever heard the Diablo Sin War Remix, you might recall a few things in the lyrics/story that give light to the fact that humans would ally with either side. And who knows, maybe there will be something to do with the Horadrim, although Deckard Cain is supposed to be the last. There are a lot of options with Diablo, too.
Basically, I say the chances for either are very high. Although I'd love to see a Diablo MMO game, I would be just as happy to see Starcraft 2 finally make it to the shelves.