I think this is an awesome idea. It would help players to get into their other roles that they never truly played with. Of course you'd need to balance out rewards vs. difficulty of the dungeon what with the potential of having people join in who haven't a clue what they are doing. I don't mean that in a negative way, but if you are somebody who NEVER tanks, getting forced to tank Aurum Vale could cause more frustration than fun if you aren't working hard for a cool reward.