Welcome to the world of current Dungeon Instances! Your description would require an instanced environment, just like dungeons. So much for the "open world" side of things.
The main problem, as I see it, is Reward. You can get gear in certain dungeon instances already, as well as minions and orchestrion rolls. Do you propose dropping BiS gear in these instances? How are these new "dungeon like" instances going to differ from current maps or raid/dungeon instances?
