Here's where the rumor comes from, from the Lore Q&A at Blizzcon:
Q: We've seen over the years the overarching dangers etc, it has been getting bigger bigger and bigger. Are there any plans to take some more bigger things down? Will we take down Alexstrazsa for example?
Chris Metzen: She's the Aspect of Life man, can you really punk her down for long? A careful way to answer this question is, obviously Deathwing is the bad guy and in the subsequent patches similar to how WotLK worked, ultimately when we develop the final combat with Deathwing to put him down, there is really good money on the fact the family will get involved. Their greatest and most horrible hour, the will perform the function they were created for. It's probably best to let the specifics breathe at this point. Deathwing is so gnarly, how do you defeat something undefeatable? Can he be defeated by any martial mean? How will the family (AND THRALL) pull this off?
Oh I just have to say it, you know Thrall is a shaman now. So what Aspect is missing from the family? Neltharion is out, the family has been broken for 10,000 years, what of the 5 aspects are you missing? Earth. So I'm just saying. Maybe that's a way Thrall can distinguish himself helping the family back together for a big win.
EDIT: And a second question from the same panel:
Q: When we first met Garrosh Hellscream he was kind of an annoying kid? Now he's the leader of the Horde. Are we going to get a Warchief out of this guy?
Metzen again: We know that the whole Garrosh thing is "contentious" and it's part of a much larger storyline involving Thrall. Thrall is the most important person on Azeroth, the Garrosh storyline has a lot to do with Thrall. Thrall is a Clark Kent type character who doesn't really know his true strength, he was pushed into leadership very young and Garrosh is used as a counterpoint to Thrall. He's a hothead against someone who thinks a lot. Ultimately the story isn't about Garrosh, it's going to be some of the best storytelling that WoW has ever done.
---------- Post added at 11:08 AM ---------- Previous post was at 11:02 AM ----------
So, really, it's largely speculation that Thrall would be the aspect of Earth, but he'll definitely have something to do with Deathwing and he has a greater purpose than Warchief of the Horde. Metzen has often said that Thrall is his favorite character (which I imagine is only partly because he voices him), so I imagine he'll be really important by the end of all this.
It was stated outright that Kalecgos would become the new aspect of Magic, though.