Got and idea brewing in my head! It's very raw and I was considering it in class, instead of paying attention to the lesson =P.
What if "Madara" isn't really behind that orange swirly mask? What if it is really some nin named, Tobi...This is all hypothetically and is my second theory to the "mystery of Madara" (ask for the first, if you'd like to know).
This man by the name of, Tobi is a scavenger of sorts, one of the most brilliant nin to ever live, a scientist who experiments with his own body, and a man wanting to become the next So6P. You could say, he is cut from the same cloth as Oro. He is very old and during his lifetime he has acquired the rinnegan (perhaps he found, then stole them from So6P's coffin), countless uchiha sharingan, including Madara's, as he died, and even the first's DNA...
With all of these items gathered and complete information of the Rinnegan and So6P's stone tablet, he knew he needed to plant the rinnegan in an uzumaki for the their full potential to be realized. However, he needed to torture this vessel, aka Nagato, to lead him down a dark path and commit all of the acts we've seen unfold.
After Nagato's body was sacrificed, not Mr. Tobi's, by summoning the Gedo Maze. Tobi then knew he could capture all the tailed beast on his way to becoming the next So6P.
He knew he needed sharingan eye's and senju chakra/blood/dna to achieve two of the six paths in order to become the next So6P. Once he gathered these two items he used them to create Zetsu by means of izanagi. He sacrificed one of his sharingans to do so, but had madara's sharingan as a replacement because he stripped him of it while madara was dying after his fight with hashirama. Zetsu allowed tobi to repair any injuries and Tobi now had the powers of madara's eyes.
However, I don't know why he's claiming to be madara? perhaps it is out of respect, since he using his eyes, or it gives him creditability, instead of some nin named tobi?
Also, if this all the case, then the sixth coffin would be madara!
Thoughts anyone? I know it's a little flawed, but I couldn't help but share =).
Enjoy, dudes and dudettes!