1. The Yakuza bosses. Did literally nothing.
2. Kiku. Did nothing. Was only there for trans representation.
3. Kawamatsu. Did nothing. Denjiro could have been the one watching over Hiyori from the get-go, and then Kawamatsu would have absolutely no role in the story.
4. Ashura Doji. Like Kawamatsu, his character and accomplishments could have been merged into Denjiro. Make Denjiro the former Kuri gang leader that Oden had to defeat (which would be fitting since he went on to become a Yakuza boss later on). Make Denjiro the one that was funding the rebels in Kuri and stole the Onigashima blueprints (which would make more sense anyways since Denjiro was close to Orochi). The only useful thing Ashura Doji did in Onigashima was stop the Oden drawing from blowing up the Scabbards, which never needed to happen anyways.
5. Raizo. All he did was have a pointless fight with Fukurokuju, and be the reason why Jack came to Zou. Oda could have just had Kaido find out that 2 of the Scabbards are ruling Zou. which would prompt him to send Jack there to take care of them. Or Kanjuro could have been the Scabbard on Zou that Jack was looking for. This would make his betrayal sting even harder since the Minks risked their lives to protect him. Also, Kanjuro could easily fill the role of being the token ninja amongst the Scabbards. Kanjuro would make much more sense as a ninja than as a samurai anyways since he barely uses a sword, and his drawings could easily be considered ninjutsu.
6. The Numbers. Complete waste of a cool concept.
7. The BMPs. They did nothing but sit on their boat the entire time. Even Perospero's presence didn't really matter on Onigashima, so he wasn't needed either. If it was actually Carrot who defeated Perospero, his presence would be justified since defeating him would be important for her character. But instead, his defeat ended up being a damp squib since he got defeated by Neko, who had no connection to Perospero, after an offscreened fight.
8. Hawkins. Did nothing but have an unnecessary fight with Killer. Just kill him off during the TS
9. All of the named Gifters.