These are just all the fights that I'm thinking (mainly hoping) will happen at the end of One Piece. I'm just listing possibilities, and this isn't in any order or anything. (Spoilers for people not caught up yet)

Usopp vs Van Augur: This one is just a no-brainer, the two of them showing once and for all who'll be the true King of Snipers. Not much else needs to be said. (Though if Usopp doesn't get an Elbaf power up then he's doomed)

Koby vs Kuzan: This could be debatable. Because of what happened on Pirate Island with Garp, Koby likely wants to inflict payback. He'll have mastered all of the Six Powers (including an improved Six King Pistol, used to finish off the former Admiral)

Koby vs Akainu: A second possibility. Based on the experience at Marineford.

Zoro vs Mihawk: The most obvious fight here. Zoro pushes his Nine Sword Style and King of Hell Three Sword Style to its limits, while we finally see Mihawk's true power. Zoro doesn't actually kill him though.

Sabo vs Akainu: I wanna leave BB to Luffy, (for now) and Sabo will take on the man who actually killed Ace himself. By now Sabo will have learned Ace's Flame Emperor and mayybe have awakened his fruit, or at least learned more powerful techniques. (also side question if Sabo learned Finger Pistol how bad would it be because of how strong his fingers are already?)

Luffy (& possibly Sabo) vs Blackbeard: We'll all be very excited when this happens. I think Blackbeard may kill Garp in a similar cowardly method as he did Whitebeard, which results in Luffy raging in Gear 5. (I have a theory which I'll post at a later date) Blackbeard retaliates with both of his awakened fruits. (Or the third fruit theory everyone says) Sabo could finish with Akainu early and join the fight, snapping Luffy out of his anger craze. Luffy will use Red Hawk and Red Roc a LOT during this fight. If Sabo does show up then there'll be a cool fire combination attack to finish the job. Hopefully they do actually kill him.

Dragon and/or Shanks vs The Five Elders: This one literally just now came to me. Depending on how it goes on Egghead this could happen. And also Dragon needs to actually do something after 1100 chapters.

And that's about it for now. I am very tired this was way more work than I thought. I might come up with more ideas another time if I think of any.