Most people I know absolutely loathe doing roulettes and literally only do them because they're the most efficient way to cap their weekly tomes. If harder dungeons were introduced with a compelling incentive I have no doubt that they would be successful. One thing that could be done is encouraging bosses to be killed in a specific way for achievements - which is one of the few things I miss about WoW. Once every dungeon achievement is completed that player can walk away with a prestigious mount, minion or title.