I like the ideas of making the late game more difficult. What if one replaced all the minions and LTs with bosses, and replaced bosses with super-bosses in a mission? Or be more bold, keep the 3 tier system and take minion->boss, LT->super-boss, boss->AV. What if a +5, +6, +7, +8, etc notoriety was created? What if there were enemies that bypass some forms of buffs/debuffs? Enemies that always hit you at set damage? The game can be made more difficult imo with those methods, but I don't know how hard it would be to program.