I'd love for these to be the equivalent of Davoi in FFXI. Entire city zones for NPC's. Imagine a city the size of ul'dah populated entirely by enemy beastmen!

Fighting your way through respawnable mini-bosses to open gates. and then the final room boss could be an instance. so each group that shows up has to pop the door mobs, victory and move through. until you reach the final door and fight a mega boss

This method allows all the areas in between to be open areas that you can grind in!