Another idea would be to reduce the bonuses perks give as you increase the difficulty. So at novice/adept, the destruction stagger perk would be 100% chance. If you turned it up to Master, you might only have a 30% chance to stagger. In fact I would prefer this option.
I don't think either of these would be very hard to implement, and I'd be much more inclined to purchase downloadable content if I knew it might actually pose a challenge.
Edit: I play on Master difficulty, btw