This could easily be accomplished if SQE ever figures out how WoW's "Phasing" worked. Basically instances would become open-world areas that are "Phased" for up to one group of players. Completing that instance would "Phase Lock" those players to a "Completed" phase, where you could run into other people, etc. Could even be some rare mining or botany nodes in the "Completed" phase for people to farm, giving gatherers a reason to complete these instances. If you wanted to repeat the instance, there would be a way to "UnLock" the phase, just like how WoW had the Reset Instances option.
But this is all contingent on the idea of Square figuring out that whole "Phasing Technology" idea.
Using this would enable them to add more and more open world areas to the ones that already exist. I would LOVE to see open world dungeons with mobs, leading down to the instance in question where you just... STEP ON IN. No zoning! I hate zones! >< I don't mind a little more travel time (Zones in 1.0 were huge, which is cool, and no zones, which is awesome, but those zones were largely boring and empty, which sucks... A balance can be struck!) if it means the world feels like... you know... a world, instead of a theme-park.
