Exactly where did they say that they have reserved spaces on the map for the add-ons?
If you're talking about the Invisible Walls, then consider what the Devs in Fallout 3 did. 3 of the 5 DLCs added doors in out-of-the-way areas that you were unlikely to visit in the first place, and those doors led to areas OUT of the map. And the 2 that added doors in plain sight were basically a door added on a wall or a manhole in the floor.
The Invisi-Wall syndrome is probably due to Dev Lazyness, but it's highly unlikly that they specifically reserved those areas for new DLCs.