I've always thought that this sort of content, if done on the world wide scale, can be a casual but fun part of a game.
Since I haven't played many MMORPGs, although I started out with UO's launch, so I've have been around quite a while, I wanted to see what has been done. And what gamers liked.
UO had treasure maps that took you to a general area, and then you used a skill to search out the actual buried location. The as you dug, there were chances for MOB spawns. The bigger the treasure hoard was, the tougher the spawn. It was a fun system to play, plus there was a lot of trade involved for treasure maps.
I haven't seen it myself, but I think a game with "keys" that drop as loot, and are marked for a location, acting similar to UO's Treasure Map system (above), usually in a Dungeon or Ruins, would be nice added content.
In that sense, "keys" could come in a lot of forms. They wouldn't have to be actual lock keys. Think 'National Treasure' and the meerschaum pipe here.
So, I'm curious about what I've missed, what with so many games that have been made over the years. What was well done, what might have been improved, etc.
I guess I should state that I'm not looking for the typical Dungeon run here, where you get keys dropped off MOBs in one room, and then use them there or the later rooms. Thinking more along the lines of new locations as an incentive to go places in the game world for rewards.