T-shirt’s with some of the mobs/bosses...
I would love a poster of all the mushrooms in the game (not just the gatherable ones)
Something commemorating the peace between Spriggans and Treants.
Buttons/Badges....’I beat the level 20 elders’, ‘I beat the level 30 elders’, etc...then you could collect them and keep them on your work/school bag or something. Fun.