Well.. after reading Artifce's post; I like his theory about Uzumaki being Rikudou's Clan. The descendants Uchiha / Senju, came forth from the Younger And Elder brother, who both possessed the powers of Rikudou, which was:
Younger Brother --> Physical Energy / Life Force
Elder Brother --> Spiritual Energy / Strenght of Chakra
Just because Uzumaki's has a lot of life force + physical energy, doesn't mean that they had to be descendants of Younger Brother? As we know, Rikudou himself possessed both of what his sons were having. So, Uzumaki Clan could be Rikudou's Clan; he had a look a like of a " Monk ", and people who were a monk normally had a clan. I'm still curious what it could be Artifce, but thanks for this theory, I like it.
Let us go further on your theory; the moment when Gedo Mazou was summoned, was because Nagato lost himself and was boiled up with anger / hatred towards Hanzou, who made him kill his best friend. As you can see; the results came forth from " HATRED ".
- Lets go back to the last day of Edo Madara; he told Kabuto, that he awakened Rinnegan, right before his death. Maybe awakening Rinnegan wasn't the thing that killed him, but something else. That something else could be GEDOU MAZOU.
Hatred and Anger are common factors for an Uchiha, especially for someone from Madara's calibur. By awakening those eyes, he felt the POWER grow inside of him and with his exitement, he got convinced that this new power was the power he needed, for owning Senju. Even thinking about Senju ( Hashirama specifically ), could be THE TRIGGER for unleashing Gedou Mazou.
By unleashing Gedou Mazou, he was slowly losing his LIFE FORCE; something he didn't had completely, because he ain't a descendant of the Younger Brother. We saw what it did to Nagato: it drained his life force like a Motherfcker..
I personally still believe, that it's a power GIVEN FROM THE HEAVEN. Madara manipulated his own body for awakening such Doujutsu, it was never granted to him. On top of that; if we have to believe that Madara " literally " gave those eyes to Nagato, then Nagato wasn't also the CHOSEN one, for wielding such Doujutsu. He could use it, because he had the body dor doing so, but it was never granted to him from the HEAVEN.
This could mean that what Rikudoú told the Bijuu's, is going to be Naruto Uzumaki. Naruto could be the one, who's going to be granted to wield the Rinnegan ( I still don't like the fact if he would awaken it.. for ONE SHOT DEAL, okey.. but awakening it for always?? )
What I'm also trying to say is; if Tobi wants to synce GM with Sasuke, that means that Sasuke needs a huge life force capacity. That's why I believe, that Tobi has somekind of plan for extracting life force out of Naruto. Tobi said that he has a suitable place for Sasuke to fight Naruto.. Tobi knows that Naruto is the child of an Uzumaki ( Kushina ), so he knows that Naruto possesses great life force. Lets say that Tobi or Madara, who told him.. knows that Kyuubi can boost the life force to another level, when it's linked with a Uzumaki! Maybe that suitable place for them to fight, is a place that drains the life force out of Naruto and transports it into Sasuke or Gedou Mazo. This way, Gedou Mazo doesn't drain the life force from Sasuke, but from Naruto.. if that happens; then Sasuke can use Gedou Mazo for getting control over the war. Only problem is Tobi's control over Sasuke.
I think that Madara knows a lot about Kyuubi, then we think. That tablet has some huge intel about everything and THATS WHY I STILL BELIEVE, that it couldn't be Rikudou who made it. These are curcial information that could only bring more pain, hatred and loss into the world and that's something Rikudou didn't wanted to. That's why I believe that the ELDER BROTHER WROTE it, so his descendants ( the Uchiha's ) would go on where he stopped; the circle of hatred. None of the descendants from the Younger Brother, could read it.. they aren't blessed with those eyes. On top of that; I can't believe that it was coincidence, that it's underneath Konoha ( secret place for the Uchiha's ).