I would like final Eggyhead to be a secret secret boss.
Ultra Egg-stinct should be his first secret boss, and after he loses he asks you for "the crystal... the staff... the amulet..." (Meaning the crystal of creation, Redis staff, and an Inkstone amulet) once you bring them to him, he'll fuse with them turning into the ultimate boss: FINAL EGGYHEAD. (Maybe you could do something similar with Inkmatas to fight Cosmilite Inkmatas)