They 'll be most likely the 2 sons of Rikudou
I don't think that's likely. How would those two people know anything that Sasuke was interested in? They've been dead for centuries. They may know some things about how the Uchiha/Senju feud started, but they certainly don't know "everything" about current events.
All the previous hokage were sealed. I don't know what that seal Danzo used does, but I suspect he can't be brought back anymore. I think there are only a few viable options:
1) Homura and Koharu, the only two remaining people alive who knew the truth about Itachi. But then again, Sasuke already knows the truth so all they would be able to do is confirm the details.
2) Sasuke's parents, or other Uchiha clan elders: They would need to be brought back with Edo Tensei, of course. I don't know what they would know that Sasuke doesn't already know. Maybe they could just confirm a few things about the fox attack and the coup.
3) Obito and Madara. But, those guys are easy to find: Just go outside and look for the place where the sky is dark and bits of the ground are floating up into the sky and there's a gigantic statue freaking out. Sasuke doesn't need Orochimaru's help to get there, and since Orochimaru is weakened, he won't be much help in a fight.
4) The kages, maybe? But they probably aren't too interested in helping Sasuke anyway, even if he does find them.
In my mind, the person who seems to know everything is Orochimaru. But clearly, Sasuke isn't asking Orochimaru to take him to see Orochimaru. It would seem like it's somebody that Orochimaru knows how to find, but Sasuke doesn't.
The whole thing is very confusing. I definitely think it's somebody we haven't seen in the manga yet, because very few of the characters we have seen make any sense.
