What about New Game + system? Start again but the game gets more harder, bosses with more health and AI more AGGRESSIVE, NPC selling items for high prices. Some special effects like: EoW more loooonger, moonlord with more hands, eye of cthulhu predicting dash, twins but with more 2 eyed linked... (The more NG+ the more difficult it becomes. It would be interesting to make more use of the end game weapons.)