Rikudo No Jutsu
When Minato first met Naruto, back in the invasion of Pain arc, he said the following:
He said the seal was set so that if the 9 tails was about to be revived, i.e upto 8 tails were reached, he would appear within Naruto's subconscious.
The 4th designed this jutsu should the 9 tails ever become a threat again. Now, Minato simply stopped the Kyuubi, but what about the Sage of 6 Paths, who fought a far greater threat to the ninja world in the form of the Juubi? Could it be that he also left a fail safe, incase the Juubi were about to be revived?
The Rikudo Sennin wanted the same kind of peace as the younger son had, he would'nt have approved of what Madara and Obito plan to do now. I think he may have left some kind of jutsu, one which may possibly allow him to meet with Naruto, before the full powered Juubi is revived.
Its true that in the 4th Hokage's case, Naruto was a jinchuriki so he could design a jutsu to appear within his psyche, but the Rikudo would not have been able to forsee the current events so how could what i just stated happen? Well if you read this:
The Sage mentions one day they will all become one, meaning he anticipated some of what has come to pass in the manga. Its possible that Naruto bumping fists with all the Bijuu might trigger the Rikudo's jutsu, this was suggested in the previous pages as well.
Just to remember, the Sage chose the younger son as his successor as he agreed with his ideology as opposed to the Elder's .