An idea for a seemingly easily implementable hard mode option popped up in my head earlier while reading the title of the "Expert Mode" for WoW thread. The mode would allow the player to up the virtual level of mobs while leveling by an amount of their choice. This would affect the amount of damage and HP a mob has to make leveling more challenging for those who want it.
Additional features could be to modify certain stats individually such as only increasing a mob's damage by X amount of levels while their HP stays the same, visa versa or a combination of the two.
It wouldn't even really be a mode, just an option in the interface that you could change and disable whenever you wanted.
I have no clue if this would be easy to implement at all, just assuming that if they can alter and have levels scale with you with you that they'd be to add +5 levels to mobs across the board that'll alter the damage and HP they have.