That's the rumor anyway.
It's a really boring rumor imo. Of the three regions explored in the original game (considering EotN and Prophecies as one), it's the only region left. Expansion 2 is visiting Elona, So by elimination of course Season 4 must be Cantha. If not then, Expansion 3!
So I can defend the rumor as easily as having made it up on the spot or as having had Mike O'Brien tell me directly. It's defendable but almost certainly baseless. But at the same time, if Cantha does end up showing its face later in GW2's life, anyone who merely guessed that Cantha would appear is vindicated and anyone that doubted is immediately a dummy.