New Difficulty Option - Time Increases Difficulty

Add a new difficulty option similar to hardcore mode option that increases world difficulty as time goes on or as you complete certain task etc.. beating one of the bosses. This would slowly make mobs have a little bit more health and maybe start spawning with certain potion effects like speed and strength the more time you've spent in a world, have the speed only slowly increasing so you wont really notice a difference until around day 250 but if you beat a boss it could give you an instant boost to the difficulty level.