Originally Posted by
paulbee
I'm thinking that perhaps there is NO That Jutsu. JIraiya could just have been referring to the Jutsu of Taking Control of the Kyuubi's Chakra. As we all know, Japanese language and sentence structure can sometimes become cryptically challenging for interpreters.
Anyhow short of the above, I can think of some wildly imaginative Jutsus. The biggest would be Naruto fully becoming the Rikudou, without tricks like stealing DNA or Eyes.
The Original Rikudou was born that way and gifted from the Gods (from Heaven) to be who and what he was. For Naruto to truly become his true successor, he would have to be likewise Gifted from ABOVE, by birth, or by earning the Right.
I don't think Naruto's lineage is sufficiently complete to produce a By Birth Rikudou transformation, but I suspect that Minato's resemblance to the SO6P and Naruto's own Heart his Nature his personality has moved him close to being like the Rikudou , which is why the Tailed beasts accepted him.
Perhaps then when the Bijuu gave Naruto their chakra, Naruto also received the SO6P's own chakra, perhaps The SO6P had given his chakra to the Bijuu when he transformed them from being Juubi Chakra into who/what they now are.
So if I were to speculate, I'd imagine that That Jutsu Would involve a Risky act that can only be successfully done with Through Kyuubi's or Rikudou's Chakra, but when done it will earn Naruto a full Rikudou transformation and status. Perhaps we might see him entering through Enma the King of hell's mouth, meeting the Original SO6P, and coming back out, fully transformed as the New Rikudou, perhaps this is That Jutsu.