Disable EDs in challenges.
Playing without EDs would be fun too and actually some challenge.
Unlocking artificer when lvl cap was 20 and we had no EDs was fun (soloing was not an option to me).
Past lives should be disabled too (or at least Epic/Iconic past lives).
All bonuses from epic lvls probably should be disabled too (including skill points/epic power/epic feats/hp/etc).
But no limit on your gear (eg. can wear ML28 stuff if you are lvl 28).
You get XP though, that includes ED xp and karma points.
Moreover, XP rewards should be probably increased or challenges drop some new matts with disabled Epic stuff.
This would make new players (or new toons) less gimpy compared to vets.
EDIT.
The word challenges should actually mean something.
Or it could be optional (like more XP for greater challenge).
The idea of disabling your powers for doing some specific quests came from other game I'm playing (there are events in which your lvls in all classes are ignored and thus you are as strong as others in your non-main classes, which is a very good opportunity to get XP in other classes).