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Many will probably disagree with me on this, and I have doubts about it myself, but I think that Tobi may NOT be Madara. I know that Tobi being Madara makes sense and no one else would fit the part (at least I can't think of anyone) but I just want to put these out there:
1) Tobi is just a mask. Any ninja powerful enough could put on a mask and carry out an attack and blame it on Madara. Uchiha Madara was an extremely powerful ninja, so it would make sense to anyone who knew how powerful he was. Another reason he could be pretending to be Madara is to make people fear him and cause the 5 Kages...