- Luffy vs Akainu: He’s definitely Luffy’s fight, he’s the reason Luffy started to learn Haki, he’s the reason Luffy has a scar smack bang in the middle of his chest as a reminder of his loss, he’s the only person in the story to break Luffy’s will and give up on being PK, he’s the one alongside Blackbeard that was named as the two big changes in the world, he’s the one at who’s name Luffy shows anger, he's the one that actively tried to kill Luffy, he’s the one that killed Ace - the brother Luffy spent a whole 7 years with as opposed to merely 9 months with Sabo lol, the brother who we saw Luffy trying to save multiple arcs upon arcs, only to lose him. Luffy is the one that has history with Akainu, both show grudges and see failures in one another (Luffy failing to save Ace and Akainu failing to kill Luffy allowing him to rise and at that point become PK). Pretty set in stone imo and anyone other than Luffy is not as satisfying both emotionally and narratively (Luffy will then go on to fight Imu - if they are a fighter in a group effort like Kaguya from Naruto, and even Oars/Kaido)
- Zoro vs Kizaru: Similar to what people have brought up that they are both the "number twos" of their respective parties (it can also be Sanji because of the whole light vs speed theme)
- Sanji vs Greenbull: Similar personalities (themes of food and women - he can also fight Kizaru? see above)
- Fujitora maybe switches or refuses to fight? If not then maybe him vs Sabo (they've clashed twice in Dressrosa/the Reverie - perhaps Sabo's ideologies can factor into Fuji switching sides?) he can also fight Zoro (both swordsmen?)
- Admirals vs the SHs is happening, from the very beginning the central conflict has always been Pirates vs Marines, and the admirals have always been a constant threat in arcs like LRR, Sabaody, Marineford, Dressrosa. Keeping them in mind is what allowed Luffy to learn Haki and become stronger to land a hit on them, his efforts and the crew's will pay off. Tertiary parties like the Revolutionaries and the Gorosei/Im are the icing on the cake that is Oda's world building which will prep us for the final war that Whitebeard predicted.