One thing that I've found myself wanting in many games is a really customizable difficulty. Many games have a difficulty slider, but each setting on that slider may have multiple changes.
What I'd like to see is an itemized Difficulty.
For Example: (note this is a super simplified example)
Say that normal mode has the standard settings. But Hard mode adds 50% to enemy HP, damage, resistances, etc and increases the AI level. Extra Hard add 100% to those values and increases the AI even more.
An itemized difficulty setting would allow the player to chose to use the Extra hard set