Apparently MMO Champion is claiming/strongly suggesting that Goblins and Worgen will be the new races in the next expansion.As if it hasn't been posted a million times before... still, I find it interesting.
I know that Pandaren will never become a playable race, so that really only leaves a few races that are already in the game as options. The other thing that Blizzard could do is introduce something else entirely, although that didn't go as smoothly as they liked when they crash landed the Draenei on Azuremyst the last time.
MMO is making the claim based on four Hallow's Eve masks they discovered: male and female versions of Goblin and what looks like new Worgen faces. This is significant (according to them) because all of the current masks are playable races. They make a good argument, honestly. I'm one of those that believe that, if this is actually true, Goblins will be aligned with the Horde and the Worgen with the Alliance. Goblins already appear to be more buddy-buddy with the Horde anyway, and the Worgen were once the Human nation of Gilneas, so it makes sense. Also, less important, but should be taken into account, it would give a small race to the Horde and a bigger/furry race to the Alliance.
What remains to be seen, of course, is whether or not Blizzard will confirm this information at Blizzcon. I'm actually hoping they do. I have one level 80, and another toon on the way towards 80. One is of the Horde, the other of the Alliance. Rerolling doesn't bother me much. I'm a dedicated altaholic. Expect to see me roll at least one of each of these to play around with them if they come true.