the Hashirama/Madara battle is kinda expected at this point. with all the hype of Hashirama, I predict Madara will give his flashback when he is about to die/sealed.
As for the SO6P, Im not sure how that one would work, and frankly im not sure if I want him to make a flashback of that battle. For one thing, the only beings to actually know of the true backstory are the tailed beasts, however since there are 9 the only way that would work is if all 9 get together and have a little trip through memory lane. The only way I see that happening is if Tobi collects the 8-9 tails, and merges them back into the Juubi. Now I dont really want a flashback of the sage because it kinda ruins the whole mystery type deal with him. The sage is the God of the Narutoverse, so a backstory on him would kinda kill his character's momentum. Little snipets of info are more than enough of him.
