Having to travel over the same empty spaces gets boring fast, and it will only get worse as the game world grows. You could have a number of exits on the maps that would be like "red exits" in fallout- they put you on the "main" map of the Metro, and you can pick a location to go to (again, like city maps in Fallout). This would also allow GSM to be a little further away from SGS then the 100 meters it is now, without having to make a myriad of new (or, god forbid, same) tunnel levels.
