How about reintroducing XP bonus for doing quests below the actual level. We had that once but that was removed.
I remember doing certain quests (e.g. crucible) with party members that were far below the actual level (e.g. a low level cleric). But that was way before certain spells were changed from bestowing a permanent effect to very short duration effect. Since then the minimum levels for various quests were raised and spells changed to be less effective or even ineffective. So the only thing of interest would be to reintroduce the the bonus for say doing a quest one or two levels below the actual quest level (depending on difficulty).