Spoilers from chapter 490 onward.
These are just two small theories I have, and neither have any proof; they're just pure speculation, though I would like to see what other people think.
Deception
This is just a basic theory. People talk about all the things he did, and all the power he had. Madara Uchiha was powerful - there is no denying that. But, is it possible that it was Izuna who took Madara's eyes, meaning that the man we know now as "Madara" is actually his brother, Izuna Uchiha masquerading as him?Kabuto's Trump.
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This is my second theory, and it works weather or not "Madara" truly is Madara Uchiha or Izuna Uchiha. I think that this sixth resurrected body, is Madara/Izuna.
Any thoughts?