Hashirama vs the First Kages
The battle that could have killed Hashirama and sparked the First Great Shinobi War. The Kage Summit once resulting in the death of the God of Shinobi, whether it was an ambush or if things devolved terribly, would be pretty historic and provides a relatively acceptable answer for Hashirama's cause of death.
Third Hokage vs Third Raikage vs Third Kazekage
The three who are believed to have been the strongest Kages of their village (even if it's because Hashirama's strength had become a myth) in a battle royale, probably during the Second Great Shinobi War. Even if it wasn't shown, knowing that such a battle had happened would have been cool. I'd include the Third Tsuchikage who probably is the strongest they've had but the Third Mizukage is a total mystery while the Fourth seems more likely as a perfect Jinchuuriki.
Jiraiya vs Itachi and Kisame
Expanding on the canon battle where there's an actual skirmish, even if it's just a few more jutsu being slung around.
Sasuke, Suigetsu and Juugo vs Kages
Expanding on the canon battle, Suigetsu and Juugo should be more capable of contending with kage-level opponents. Sasuke has a lengthy rematch with Gaara, Juugo brawls with A and Suigetsu faces off against Mei.
Onoki just chills while keeping an eye of Danzo or something.
Kurotsuchi vs Edo-Deidara/Darui vs Edo-Kakuzu/Chojiro vs Edo-Mangetsu
Three battles during the Fourth Great Shinobi War to show that these individuals are capable of becoming the Kage of their villages. Kurotsuchi and Chojiro never had a fight of their own; the former can capitalise on her history with Deidara, the latter can defeat presumably the strongest of the old generation of Swordsmen and both deal with traitors to their villages. And Darui gets a fight that relies on his actual skill and power instead of the gimmicks of the Treasured Tools
Hanzo vs Tsunade
There's a history betweem Hanzo defeat and named the Sannin, and Tsunade has a reputation for creating antidotes to all kinds of poisons according to Chiyo. Despite her age Tsunade should be a lot stronger than she had once been in her youth thanks to the Strength of a Hundred Seal and its jutsu so she could him on without her teammates unlike before. She would actually be a lot more effective in showing how Hanzo had changed in the years since she was someone who'd fought him at his best.
And Tsunade would have had an actual one-on-one fight in Part 2 instead of only being a teammate for the Madara battle which they all lost anyway.
Mei vs Gengetsu
Instead of Gaara taking on two Kages, Mei gets to fight and surpass her predecessor just like how he had defeat his own. In canon, she remained the least developed and least impressive of the Five Kage.
Jinchuuriki vs Akatsuki