I like this idea as well. There could be some additional factors/elements put into play:
- everything about the fight is the exact same, except the hits and damage you receive (unless they are suppose to be an insta-wipe like enrage) would be 50% or less.
- no loot
- the "training dungeons" come out after 4-6 weeks of the original release
- zero (0) restrictions on how many times you can run them