Just wait it out and we'll find out eventually
It is obviously Madara. All doubt was destroyed on the identity of this man once you realize Madara has been alive far longer than any other man on the , and knows more about everything than anyone else. His usage of the release of the Edo Tensi confirms that he must obviously know everything. You even see him in a panel left to sasuke when he is talking about meeting the man who knows everything.
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i think it is the SO6P 2 sons.
My guess would be one or all of the dead Hokages.
Sasuke is questioning the path he's taken after his second encounter with Itachi, and he wants to know why Itachi would sacrifice his own clan for the sake of the village. The First Hokage, or really any of them, would provide perspective on that point of view.
Of course, the first four Hokages are currently sealed in such a fashion that they cannot be resurrected. That's where Orochimaru and his scroll come in. Orochimaru must have figured out how to break the Dead Demon seal in order to recover the full use of his arms.
ima go with madara, the argument you guys are bringing doesn't really bring an argument, Madara know's everything about the clan and about the village he was there when the leaf village formed, and he was the leader of sasuke's clan. so yeah madara makes sense. oro probably knows where madara is ...sasuke doesn't even know what madara looks like. he thought tobi was madara.
on a another note can see how it can't be him, but someone who's died(madara) or isn't human but then again , there going somewhere which is familiar to sasuke.
1) Fugaku : Fathers are use to tell everything to their son.(Kakashi's words to Naruto after Naruto got beaten up by cloud girl)
2) Elders of the Leaf (Trash of secret holders) : The long last remaining. (who will be cast out after speaking the truth)
3) Obito and Madara : Last option. (Madara is the main key to everything)