When I heard about challenges and mini-games, in my mind, I thought it would be puzzles, like the Abbot... not just timed quests.
I would love to see more challenges that actually challenged you to do something other than kill stuff. The Abbot puzzles are a great starting point- you have the arcade action of the asteroids, coordination of players in ice and goggles. The Monastery/Shroud light puzzles. The Crucible swim. The Pit jumping pipes. Prove your worth with the alternating ladders comes to mind as well.
I'd love to see more things that challenged player skills instead of just moar dps. They could also be great training.
These wouldn't have to be level based since player skill and not player gear would be key in completing them.
Just a thought.