I am going to explain my predictions for what each Hokage will talk about.
Hashirama- He will talk about his battle with Madara and how he did not want to kill him because he just could'nt because of their relationship. He will also tell them Madara's weakness. Also he might know Madara's Moon Eye Plan and tell them how to stop it.
Tobirama- He will talk about the reason why he created the Edo Tensei jutsu and how Orochimaru knows about it. Possibly he was controlled by Madara to create the jutsu or he was jealous of his brother's success and wanted to create a jutsu of great feat.
Hiruzen- He will tell the story of the Uchiha and will talk to Sasuke about Naruto and compare them with Orochimaru and Jiraiya. Perhaps Hiruzen will talk about his prime and what amazing things he has done and Kishi will give us a reason for as to why he is such a a great shinobi.
Minato- He will talk about Kakashi and Obito and when he figures out the man he fought with for the nine tails was Obito he feel horrible, but then he will tell the reason why Kakashi killed Rin. He will also talk too Sasuke about freindship and how he should be friends with Naruto again.
Maybe, just maybe, each of them will give an item for Naruto and Sasuke or tell how Naruto and Sasuke can get the Rinnegan. or Not.
I also wonder if all 4 Hokage will enter the battlefield. I don't think they will, but it's a possibility.
Hope you guys enjoyed it.