Having side characers/villians fight each other. One thing I dislike about One Piece fights is none of the strong villians can use their Devils Fruits creativly, because they always fight Luffy/Zoro/Sanji, people who use a brawling fighting style. Some fights I would like to see:
Buggy vs. Foxy
Ace vs. Blackbeard Crew
Aokiji vs. Ace
WhiteBeard and Shanks vs. 2 Other Admirals
Mihawk vs. Doflammango (Mihawk can defeat Doflammango to make himself look strong)
Smoker vs. The Bear Warlord
Alot of these problably won't happen but I think they would be cool fights.