If there were a secret boss in every level they wouldn't be secret, you would call them hidden bosses. I would say there should be a secret boss in every original world, that could only be faught if you met certain conditions, like the boss in twilight town would be a powered up version of the "Lightside", and after you beat each of the bosses, you get a special move or ability, like if you beat that boss, you get the move Holy. That would be awsome!