I see alot of people counting Rikudo-Sannin as a potential villain or even the final Villain or even counting his feats as those of a real shinobi, but i think people are missing the point of Rikudo-Sannin and the reason why he was introduced into the manga. Rikudo-Sannin was thought to be a myth by everyone in the entire naruto world only really Jiriaya and Madara knew the truth and that was after they met Nagato and/or Pain. Jiriaya came to the conclusion that Rikudo-Sannin must be real because of the existance of Nagato and it seems Nagato used his own existance to prove that Rikudo was real as well. But how much of the legend is fact and how much of its fiction? We have stories like Rikudo-Sannin created modern day ninjutsu and was the one who actually made the moon. Now this may be true and Rikudo did make the moon but some how i doubt that the naruto world could exist before their actually was a moon and that Rikudo was able to actually place the moon into orbit around the naruto world. What this instead tells me is that Rikudo-Sannin had fantastical powers that escaped even the imaginations of the people who lived during that time and thus his powers became embelished. It also tells me that Rikudo-Sannin did in-fact face some one during that time that made him get serious enough to use Chibaku Tensi on a higher level then Pain is even using on KN6 or even the one he was going to use on KN8. Kishi is telling us this legend of Rikudo-Sannin in bits in pieces because he is trying to show us what Naruto's going to have to live up to. Rikudo-Sannin was likely the destined child of his time. He brought a revolution to the Ninja world by teaching the foundation of modern day Ninjutsu. This revolution brought about the start of the age of Ninja for better or worse. Why did Rikudo found ninjutsu though? I think its likely to assume that Founding Ninjutsu and teaching other nations how to protect themselves was Rikudo-Sannins attempt at achieving global peace. Now we don't know if it ever worked or even if this is for sure what Rikudo was trying to accomplish, but in the end Rikudo's actions and the enemies he fought to achieve his goals made him into a legend. Rikudo became a legend for starting the way of the ninja and protecting its foundation with his unique powers. Rikudo clearly fought an evil as great as Pain, Madara, Kyuubi, Sasuke, etc... during his time and the power he used to defeat that enemy(Super Chibaku Tensi or whatever) was so great it earned him the myth that he created the very moon in the sky. Now we have a current day hero who is starting to become a legend in his own right. Naruto doesn't have the Rannigan but he is the legendary host of the strongest of the nine demons. He master the skills of the ancient toad hermits earning himself the title of Sage and has defeated countless skilled opponets. He is the child prophicized to bring about a revolution and to learn to control the nine tailed fox. Naruto posses jutsu on unprecidented skill level and power like FRS a technique not thought possible. So what is Rikudo-Sannin's role in the manga. Rikudo-Sannin is naruto's ultimate benchmark. Naruto's true goal and final destination at the end of the manga is to reach the legendary status of Rikudo-Sannin. To find his own answer to bring peace and to produce jutsu of such legendary skill and power that 1,000 years later people will still talk about them in myth and legend. Before we thought that Minato was Naruto's ultimate benchmark or Shodai or all the hokage. But Naruto's True benchmark is to live up to this prophecy of the Toads and to reach the same heights or legendary status as Rikudo. Which begs the question how will he answer the call to peace this time. I think it would be quite poetic if Rikudo the first revolutionary shinobi thats talked about in myth and legend brought about the great age of Shinobi and taught the way of shinobi and the second great revolutionary shinobi brought the begining of the end of the Shinobi way. Just my two cents on Rikudo's role in the seris and naruto's connection to him.