Sage of the Six Paths: Crazy Ideas
We've seen that the ten-tailed beast has an eye that looks like a combination of the Rinnegan and the Sharingan (Chapter #467). We know that the Sage of the Six Paths had the Rinnegan, and his descendents through his older son had the Sharingan (Chapter #462). We've also seen in the invasion of Pain that the beasts and bodies under Nagato's control also displayed the Rinnegan. This is because they are embedded with chakra receivers, which Nagato originaly obtained from the Demonic Statue of the Outer Path.
We know that the Demonic Statue of the outer path embedded those chakra receivers into Nagato's back. There isn't any evidence that Nagato created the statue, only that he could summon it. We also see the statue being used to seal the tailed beasts, a purpose Nagato would not have been aware of before the end of the second shinobi war and his meeting with Madara. In fact, he wouldn't have known anything about the ten tails before reading the tablet at the Naka shrine. Could Nagato have been able to enter the Naka shrine and read the tablet there before all the Uchiha were massacred? Before the massacre, they would have defended that place, or made note of intruders. Nagato must have gone to the shrine after the massacre, and therefore after the first time he summoned the Demonic Statue.
We know a few important facts already. Let me recap:
1) The sage of the six paths had the Rinnegan
2) The sage of the six paths used some unknown technique to separate the ten-tails into the other 9 tailed beasts
3) The Demonic Statue of the outer path embedded the summoner (Nagato) with chakra receivers
4) Creatures and bodies with embedded chakra receivers, under the control of the Rinnegan user, also show the Rinnegan (and have a shared field of vision). We don't know if this happens automatically or if this is part of some other additional jutsu. We do know that Naruto didn't get Rinnegan when he was stabbed repeatedly by the chakra receivers, and when his body was being held by Nagato's chakra. The body being controlled might have to be dead for the rinnegan to appear. If Nagato can do it with Pain, the Sage could probably do it too.
5) The ten tails appeared to have the rinnegan (or something similar to it).
6) The 9-tailed fox mentioned the Sage of the Six Paths when Naruto absorbed its chakra and sealed the fox's body away (Chapter #499)
7) The Sage of the Six Paths separated the ten-tail's body from it's chakra, and sealed the body inside the moon. He sealed the chakra inside himself, and eventually split that into 9 beasts (Same as how the fox's chakra was separated by Naruto)
8) The demonic statue had the ability to kill by taking the soul (chakra) from the victim.
9) Madara lies.
With this information, I think we can make a few educated guesses about things we don't know yet:
1) The Sage of the Six Paths could have created the Demonic Statue to deal with the ten-tails. If the sage had the Rinnegan, he might have needed the statue to help use or amplify his Outer Path life/death powers.
2) The ten-tails was under the control of the Sage of the Six Paths, and was possibly already dead when the sage took control of its body.
3) The Sage used the ability of the Demonic Statue to kill the ten-tails by stealing it's chakra and sealing it inside himself.
4) With the massive amount of chakra, and the Demonic Statue's chakra receivers, he could have been able to animate and control the body of the ten-tails. This would explain why the ten-tails appears to have a Rinnegan.
5) The Sage could have pretended to fight the ten-tails, which he was controlling already. He could have sealed the body of it away in the moon, and using his fame and power could have essentially conquered the world.
6) If the Demonic Statue can seal the nine tailed beasts, and fuse them back together into the ten-tails, maybe the statue also had the ability to separate the ten-tails into the 9 beasts. If that's the case, maybe the statue is what the Sage used originally to do it.
7) The picture of the moon's eye plan with the sharingan on the moon looks like the same eye as the ten-tails beast is shown to have (both look like a combination of the Rinnegan and Sharingan), not the same as either a regular Sharingan or the normal Rinnegan (Chapter #467).
Now here are the really crazy ideas for you:
1) If the ten-tails had an eye that looked like a combination between a Rinnegan and a Sharingan, maybe that's what the Sage's eyes looked like too, since the Sage could have been controlling the ten-tails' body using the chakra receivers from the Demonic Statue.
2) If so, the sage could have had the same powers of the Sharingan, including Tsukuyomi.
3) The sage could have tried to enslave the world by creating or enslaving the ten-tails to rule by fear.
4) When that didn't work, he could have turned himself into a savior by "defeating" the 10-tails and turning himself into a hero.
5) If the ten-tails had a sharingan, and was controlled by the Sage, the ten-tails' new eye could have looked like a combination of the Sharingan and Rinnegan. If that's the case, the Sharingan is part of the ten-tails' body, sealed into the older son and his descendants.
6) If they have the sharingan, either the Sage or his oldest son could have been intending to use infinite tsukuyomi to enslave the world. That could be why the moon was created in the first place. If so, either the sage or his oldest son was really evil, and the youngest son was good.
7) If the Sage can control dead bodies, maybe the "sage" that died was just a puppet like the Paths of Pain, and the real Sage did not die (or did not die in the way the myth says).
8) The whole story of the Sage could have been a lie, told by the younger son who rewrote history to make his father look like a good-guy. The sage could have really been an evil person who tried to use the ten-tails to enslave the world.
...And here are some very crazy questions we might ask:
1) What if the Sage created the ten-tails?
2) What if the Demonic Statue always existed, with the ten-tails inside it? What if the Sage unsealed it?
3) What if the ten-tails was not evil, as Madara claimed? What if the Sage was not good, as Madara claims?
4) Could the Sage have been using the ten-tails for evil, by controlling it's chakra with the Demonic Statue and controlling it's body with the Chakra receivers?
Some of these ideas are a little far-fetched, but with the limited information we have now none of them are impossible. It's interesting to ask questions about things, especially the things that Madara says because we know he lies. Any of these things would make a really cool twist in the story later.