You have to think hard it has to someone from his time (could be a new character) my theory is that it was the so6p elder son who is using madara as a pawn.
Like with Sasuke's case where he needed Itachi, I think Madara might have needed Izuna. That´s the only thing I can think of, but even if he´s 'unreasonable' like Tobirama says, most of the things he said about the world are pretty damn accurate.
It's possible he had help he had followers on the DL and he had people in other villages..anything is possible and till Madara says how he didn't die be it by a Genjutsu like many believe or with help..we jus have to wait and see