I havent posted a theory from a very long time but I will try...
I came up with this while reading some manga...So there's been something that was always bothering me...when Madara was telling Sasuke the truth about Itachi, he said something very interesting...
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So basicly he says he is still alive because of his unique chakra...Now in the following link Kushina says something very interesting.
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So we know that the people from the eddy village had very long lives, but Kushina says something more...
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So here Kushina says that she had very special chakra and we know for fact that she had long live as she originates from the eddy village.
So who else has very special chakra and long live - Uchiha Madara. And as we know that the Uzumaki are related to Senju and Senju to Uchiha, why now Uchiha to Uzumaki ? My point is, I think Madara is still alive because he is a distant relative to the Uzumaki clan and the eddy village. - if we just think about it, Uzumaki have long lives, Madara too. Uzumaki have special chakra, Madara too. - I can't think of better prove than these ones.
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what do you think ?