Theres plenty written about why this wont work already. Though it might work better for quests/instanced content monsters that only you/your party can fight so as to keep them challenging even if someone has max lv/rank. They already have a system like it with the leve system where you can choose the difficulty which alters the monster's levels though it has it limits. They could implement something like that with said content which improves the rewards on higher difficulty settings.