It's fairly simple. Add a little slider bar in the options that allows mobs to hit you harder. 25%-100% should be good enough for its purpose. The slider bar would add no benefit and is purely meant as a means to add challenge to the game. I doubt it would take much resources to implement and would solve the problems or those complaining about EH and EN being to easy while allowing for hardcore people to take it to the next level in EE.
If they wanted to get fancy they could also add damage reducers(on the players damage) and save reduction. Looking forward to reading other peoples thoughts on this.