Open world dungeons would be nice... Just give them a system similar to Treasure Maps...

Lets say we get a Garlean Stronghold open world dungeon, you can go there, kill some mobs, they drop a map (and now wont again for 18 hours), you decipher the map, find the treasure chest, it's locked and requires <random> key, various bosses (not FATEs...) in the zone drop specific keys, kill that boss, open chest, profit.