It's an interesting thought, but in the end this theory just does not add up. Here are the reasons why: the masked man from Akatsuki was recognized by Kisame Hoshigaki as one of the former Mizukage, though it is unclear which. If he were really Tobirama, why would he go to such lengths to become Kage of another country, for undisclosed reasons entirely?
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Tobirama was already the Second Hokage of Konoha - for what purpose would he even need to travel to Kirigakure, and become their leader? There was no time in his life to pull off such an enormous task anyway. He was still Hokage when he sacrificed himself to save his squad; was he really Mizukage at the same time? Extremely unlikely.
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Next, this mysterious Akatsuki figure has the Sharingan, as we've seen it many times through his mask. How did Tobirama get the Sharingan? For example, refer to this chapter, or many others, for evidence.
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Continuing on, why is Tobirama (if it is indeed him) collecting the tailed beasts? Aside from "being complete," which does not match up with any event in Tobirama's life that would drive him to do such things, he was also a Hokage of Konoha, and thus had deep loyalty to his village. Why would he plot against his own village?
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Also, if the masked man, the mastermind of Akatsuki, was responsible for creating the organization, when did Tobirama bring it forth into the ninja world?
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And lastly, if Tobirama is this masked person from Akatsuki, clearly still alive, how was Tobirama himself summoned by Orochimaru when fighting Hiruzen Sarutobi, the Third Hokage?
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Would Danzo not recognize his own teammate (voice, battle style) when confronting him, both during the Uchiha massacre and during the present?
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There are too many inconsistencies with this theory. It was an interesting idea, yes, but it just doesn't work. One thing I don't understand, though, is why people find it so hard to believe that Tobi is Madara. Numerous characters recognize him as Madara, and there would be no way for it to be anyone else. Who else would possess such extensive knowledge on Konoha's founding?
Who would know the truth about Itachi and the Uchiha massacre, the Sage of the Six Paths, the origin of the Uchiha and Senju clans, and all the other people he's confronted during his lifetime? Perhaps Kishimoto has a giant twist planned ahead in the story, but for now Tobi is Madara unless stated otherwise.