How about a +20% XP bonus for every level under the base quest level up to 5 levels under for up to +100%? (or 10% to 50%... some bonus, whatever)
I've heard talk of under level xp bonus in the past. Did this used to be in the game?
Seems like an easy way to give more of a challenge to the "it's too easy" crowd.
What are the drawbacks?