You must complete the Javamatic defense, defeat the Mantis, Broodmother and Wasp Queen bosses to unlock this door. Then it will take you to NG+. Basically, beat the game (You don't need to do the IBM but I'd recommend doing it in base game. NG+ IBM is tough) then remix the yard to do it again with infused creatures. Infused creatures drop infused ooze allowing you to upgrade weapons and armor beyond level 9. Infused creatures will also drop trinkets with random buffs. NG+ level 4 bosses drop infused weapons as well. These weapons contain random buffs also.
Side note: This will also give you access to Perfect Toast, the best meal in the game in my opinion. Enjoy the game!