I'm sure that many of you didn't see the plot twist of Chapter 559 coming (many thought that Izuna was in the mysterious coffin, and not Madara), and after seeing Chapter 560 the mystery shroud only thickens.
At first, Obito came to mind, however there were still too many questions to even make him remotely-possible - How could he know as much about Madara as he does? How does he knows about the Sage of Six Paths and the Ten-Tailed Beast? From where he knows Nagato and "motivated" him to create the Akatsuki (according to Tobi's claims)? How he acquired Hashirama's DNA?
So how about Madara's brother, Izuna? He knows Madara's history, at the same age as Madara, said to be nearly as strong as his brother, and it seems reasonable to have the motives to destroy the Leaf Village and pacify the world (mainly the Uchiha/Senju struggle). However, given the actions to the resurrected Madara is Chapter 560, it's no longer seems feasible to think that Tobi is Izuna. The resurrected Madara has the Eternal Mangekyō Sharingan, which would require the real Madara to implant Izuna's eyes, which would leave Tobi (assuming that he's Izuna) blind (there's the possibility that Izuna transplanted other eyes but that seems a bit far-fetched). Moreover, Madara the "switch" between "eye-modes" (Sharingan -> Eternal -> Rinnegan) whilst Tobi was only seen with the regular mature Sharingan, and transplanted Nagato's Rinnegan instead of "evolving" his Sharingans.
I see two strong possibilities (and many more less-then-strong ones):
A. Tobi is from the Uchiha clan, with close enough relations to Madara and/or Izuna prior to their deaths.
B. Tobi is NOT from the Uchiha clan, but still somehow knew about Madara's past activities (I'm more inclined to this option)
Further more, Tobi can use Izanagi which require both Uchiha and Senju DNA, and seeing how Danzō was have to utilize Izanagi despite not being an Uchiha, and Tobi "bleeds" white substance (like Zetsu) and can replace/regenerate lost limbs raise the possibility of the second option. Also, Tobi never seen using EMS, keeps a lot of replacement Sharingans, needed an outside source for the Rinnegan instead of "evolving" his own's, the resurrected Madara (I'm not sure if using "real" Madara would fit, considering how every is jacked at this time) somehow knows about Kabuto and Tobi, was surprised that they managed to read the Uchiha Tablet (a feat associated only for the Uchiha), knew about Nagato and his Outer Path: Samsara of Heavenly Life Technique.
Then again, we're only (as of this posting) 2 chapters into the twist, and the content of this post would become obsolete, either by being correct, completely off, or any other in-between. Post your thoughts!