I have been asking myself for a while and my friend noticed it too:
Madara vs Hashirama hasn't been shown. I mean, we don't know a thing about this battle, just the place, and how did Madara "die".
Orochimaru and Sasuke are gonna revive someone soon, from the Hidden Leaf.
Sasuke and Naruto had a fight, it has been shown. They will have a fight again, everyone can tell this.
What if Madara and Hashirama get a 2nd chance?
I mean, Sasuke>Naruto back in the first fight, and Hashirama>Madara.
Now it is Edo Madara, with Mokuton and stuff. I assume Edo Madara=Hashirama now. I also think that Sasuke will get a power-up during the trip with Oro, so Sasuke=Naruto.
I think that Kishi based the story on two equal people the will fight. This is why when Naruto gets a power-up, Sasuke does too. The second fight with Hashi and Madara will give the opportunity for us to see the first fight too with flashbacks during the fight.