Additional Information on Mysterious Man Identity
I was also looking into this in depth earlier today, in a sense I do believe he/her has some relation to Hagoromo Ōtsutsuki. I particularly pointed towards a Senju ancestor as they had the sages body - life force and physical energy.
I determine this because of a few reasons.
A.) The life force surrounding the 'mysterious man' was potent in chakra/physicality; similar to that of the ancestor of the Senju clan.
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B.) I thought it was either a relation to the Senju or Uchiha due to previous events; Madara died when he fought Hashirama but there was one who oversaw the fight and in my opinion helped him.
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This to me seems like an indication someone had witnessed the deathbed of Madara and fled; as soon as Hashirama left he recovered Madara and Hashirama's DNA.
C.) It seems weird for me to conclude it was a 'Senju' who had done this but for example say it was the actual decedent of Hagoromo who's life-force and physicality was respective of his could survive this long and keep his youth
(similar to Tsunade).
Why? Because he wanted to follow his father's ways of peace, and in his father's predictions he said there would one day be a world saviour
(Naruto). The son knew this was the path to peace and knew for that peace to be realised Madara had to go about his plan.
D.) It can't be Kabuto as he can only escape if he accepts his fate and change. Even if he did then not even Orochimaru and Karin recognised this 'strong' chakra. Kabuto lost his transformation as he required Orochimaru's chakra which he took back.
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Orochimaru nor anyone were alive while Hagoromo's children were alive
(Uchiha & Senju Ancestors).
I could make a theory on this in depth and provide further facts but this is your theory, I don't need to create another, hope I helped your theory in some way. If I did you can add this in if you like and credit me. Otherwise good theory.
