I think adding something to the Challenges, like after beating the bosses 3 times you can replay on a harder difficulty for double the rewards, would add something to this game mode.
I would even say that if you do choose to replay the challenge on a tougher difficulty you essentially lose your 3 loot dice and have to earn them all over again.
Or a Hardcore mode, where you lose your loot dice if your party dies. Giving players a choice after each boss whether they think they can make it past the next boss or should they just take the dice they have and run.
Just something I was thinking about to add some flavor to the lower tiered challenges to keep them interesting