I think I'm starting to understand more about Hashi's cells. Remember how Naruto was able to effect the wood style with his chakra after he went into his newest form for the first time? Well it seems Hashi's sells are able to feed off of chakra and nullify it. That means most jutsu wont work on Hashi (cept Taijutsu) because he can restrict chakra at all times due to his cells. With that said I think that Hashi cells and wood style emit Natural Energy letting him use Senjutsu at any time, in other words he is constantly in Sage Mode due to his cells unique powers. That would explain his immense powers as well as confirm that he is the strongest Shinobi ever. (not counting the Sage of the Six Paths)
- His cells absorb and restrict Chakra.
- He emits Natural Energy.
- He's is constantly in "Sage Mode".
With that said he has all the qualities of the Sage of the Six paths "Body", while Madara has all the qualities of the Sage of the Six paths "Eyes".
Since Madara now has many of the qualities of Hasirama (due to his cells), Madara is the closest anyone has become to being the next Sage of the Six Paths.
Now lets consider for a second what combining these two traits together would create, basically we can now take a guess at how powerful the Sage of the Six Paths really was.
Eyes - Allowed him to use all five Chakra elements, meaning he was also able to use all Kekkei Genkai, and Kekkei Tōta we currently know of. This also made him a master of all Genjutsu and Ninjutsu.
Body - Assuming that my theory is correct about Hashirama's cells, then the Sage of the Six Paths constantly had a balance of the 3 forms of energy. He was able to absorb and restrict chakra, and was in Sage Mode constantly. This made him a master of all Taijutsu.
In short, the Sage of the Six Paths was the true "God of all Shinobi" and becoming the next SotSP is basically the same thing as becoming a god.
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Now for another theory. I believe that Oro will be the final villain of the Narutoverse as well. Currently Oro has a body composed of Hashirama cells which is far more potent than just injecting the cells into his body. If Oro manages to get the Rinnegan from Madara, Obito, or Nagato (he still has one left), then Oro will be even closer to becoming the next SotSP than Madara.
I believe that after Madara and Obito are defeated, Naruto will become the next Hokage, Tsunade has to die for it to happen. His first duty as Hokage will probably be setteling things with Sasuke, defeating him and bringing him back to the village. Orochimaru will probably go into hiding again, secretly working on a new plan.
Years later Oro will return with the Rinnegan and mastery over both his new eyes and his new body with the goal of becoming god. Naruto (as hokage) and Sasuke will have to work together to defeat him, since combined they also have the powers of the SotSP. Hopefully this wont lead to any "Fu-sion-ha!" moments.
Anyway that's my theory.