I was wondering that Hypothetically Kakuzu could steal Hidan's heart since Hidan doesn't need it and then use that heart indefinitely since it can never be destroyed or lose function rendering Kakuzu even more difficult to kill than before, what do you think?
Stealing Hidan's heart wouldn't matter because what makes Hidan "immortal" is his Jashin rituals. As long as Hidan keeps doing the rituals, Jashin will keep him alive. If he stops doing them, then he loses the immortality. Kakuzu wouldn't need Hidan's heart for extra immortality, he would just need to practice Jashinism. The only thing taking Hidan's heart would accomplish is give Kakuzu whatever chakra nature Hidan has.