I would be happy if they added some open world dungeons to explore. Multi layered caves and caverns or abandoned ruins and castles would be good. Throw in some random chests and keys dropped from mobs along with NMs and pop mobs linked to quests to give you a few reasons to explore them.

I love that feeling of stumbling across a random hole in a wall and wondering what's inside, then going in to discover it's a massive labyrinth with nasty beasties and treasure to be had.