While naruto was training with jiriaya... jiraiya stated that naruto was the only one with enough chakra to master That jutsu which is the jutsu that even the Fourth Hokage couldn't master...
That Jutsu is the reaper death seal
I believe that Minato created "that jutsu" just in case the Kyuubi gained control over his wife because she was an jinchuriki.
if the fox took control of his wife he would save the village and his wife by sucking the fox chakra out of her and she would be free from that burden of being a jinchuriki but there was one problem the reaper death seal would kill and suck him in as well (thats why it is incomplete) .. I believe that minato created it and also developed the jutsu with the third hokage . And i am sure minato thought he would have been able to master the reaper death seal before he actually used it. My point is that naruto will learn or already know the reaper death seal and he will be able to perform a complete version of that jutsu and actually be able to pull souls out as well as seal them without getting your soul trapped for eternity.
I believe i am right here, what do you think