THEORY - "the one who knows everything"
Could the one who know everything perhaps be...... Hiruzen Sarutobi?
I don't got any relieable facts, so it's a very unreliable theory - but i thought i would share it with you guys!
What does sasuke want to know? EVERYTHING behind the Uchicha massacre.... we have heard about the uchicha massacre from many different people; Itachi, Danzo and Tobi (possible more, can't remember).... But we haven't heard it from Hiruzen.
I mean.... we actually don't know much about hiruzen - kishi killed him off with an open-ending, therefore not really explaining his background in his prime or his fully explained relationship with Orochimaru OR his point of view on the uchicha massacre....
The importance of the scroll isen't Orochimaru, Sasuke just need Orochimaru to do something for him, something that Orochimaru has been involved in - Orochimaru was indeed involved with Hiruzen - he knew alot about Hiruzen, so perhaps the scroll is related to him!
(note: Hiruzen knew alot about the uchicha massacre which could give Sasuke some anwsers + i believe he knew quite alot about Hashirma and Madara + Hiruzen knows what a "clan, village and shinobi is" - Sasuke apparently doesn't)
Just a thought - I KNOW HIS FREAKING SOUL IS SEALED - but, but when talking about Orochimaru - ANYTHING is possible!