How about this kind of dungeon:

- no timer
- not instanced
- monsters respawn
- different floors
- you have to kill a nm on each floor to gain access to the next floor
- on floor 3, you'll get a key for a special door on floor 1, so you have to go back to this floor (so different parties will meet each other but wont steal nm kills)
- you can "save progress" like every 4 floors

And finally, it's not really a dungeon, but a tower.... and its name: crystal tower