Think its Madara but dont see how since he's not "at a place he knows well" maybe its the 1st at The Valley Of The End where he fought Naruto =P or maybe the TB since they have been around for so long sure they all know something and Sasuke has been in front of Kurama when he was sealed up making it a place he knows well since he just rudely entered without asking and didn't get lost or anything. XD (although according to the interview thing i've seen around here they are supposed to meet during the war so maybe this is why they meet =P) lol no i honestly have no idea
It's most likely the 1st Hokage, all the forshadowing is there and Orochimaru would help unseal him from the demon seal to get his hands back which has also been forshadowed so thats my guess.
The only person I can think of is Danzo, seeing as he was involved with a lot of the leaf's actions. Won't be surprised if it's someone completely new though, as it would be a decent time to introduce a new character in my opinion.
everyone over looks the obvious. the 2 people that know everything are the 2 advisors to the hokage. the other 2 people susake wants revenge on. would make sense with tobi being obito