It's still unknown if they will actually expand the area of Tyria that the in-game World Map shows us, but there's absolutely going to be new zones. So far one is known, called Verdant Brink, part of which has already been playable in a number of beta events. Chances are there will at least a few others.
Most of the new maps are expected to be in the Maguuma Jungle/Tarnished Coast area. But since it's likely that the story will include much of the world, there's at least a possibility that Heart of Thorns could include new maps in other regions as well.