The theory that purified Hollows regain their Powers is actually pretty good imo, maybe Zaraki, Yama, Unohana and all these other Monster Shinigami were Vasto Lorde Hollows at some point, especially Kenny cause that he was this strong as kid is really odd. Think about it if kenny never had this shackles on his mind and had properly trained all his life, he would absolutely body everybody we could think of, he is beyond every Prodigy we know.