We've had hills and mountains, rivers and beaches, forests and meadows, I think we'll see a new biome. Swamp or desert has potential because we've already seen those biomes in-game, just not as a farm type. I think if he was going to do a swamp, it would have been released in 1.6 with the green rain event. Therefore, I'm voting Desert.
It will probably have some sort of flavor attached as well. I'm trying to think of features that would match the existing desert map. Plantable palm trees? A larger mushroom (or fruit bat) cave? (A mini casino?)