One of the ones who know everything is Tobirama.
As the title says it could be Tobirama because;
1) We don't know much about him, while he is a very important Senju.
2) Due to 1, there should be some explanations about him.
3) We just know that Tobirama uses water perfectly and space-time jutsu which is irrelevant now and that he is the creator of edo tensei which is useful.
4) Probably, Sasuke wants Edo Madara to be vanished as Itachi's last will.
5) We know Tobirama is sealed forever.
6) We know a scroll which can turn the waves in the war.
7) Due to 4,5, and 6, Sasuke revived Orochimaru to find 'the ones'.
8) Probably, Orochimaru does not know edo tensei in full detail, we know from that there are some guys who know everything.
9) Due to 3, 4 and 8, Sasuke needs Tobirama.
10) Due to 2 and 7, Tobirama will get a place in the manga.