Add in a bonus sidequest mission, far in the desert. Have it require 201 gold stars in NewGame+. Title it 'Slak's Shoes', Slak eats desert mushrooms and wakes up there for a trippy battle.
Have it spawn every boss in the game at once coming from different lanes, including Zelemir and the final boss on portals. The reward: Slak's Shoes. An armor equippable only by Slak, which grants +99 Speed and +99 Attack.
Alas, he still cannot wear armor. =(