The idea is to have a low visibility dark map, where the player can only see the environment of the 5m radius (imagine your toon is holding a torch).
Map will be a kind of maze (random everytime), where any kind of mob from any past expansion can spawn and the bosses in the different rooms of this map will also be random. (imagine spiders dropping from above, cryo worms coming out of the ground and you cant even see whats next!)

Add hidden rooms that gives buffs and treasures(gold/order mind jewels/crafting components/etc..)
You can also add special rare bosses in these mazes, to crank up the difficulty you can disable resurrection :P.