Squard! AKA the guy who betrayed Whitbeard at Marineford. He has this big, iconic moment in the manga. Then we really never get to see him do anything or the fallout for his actions afterward. He could play a much larger role in the story than he currently is. There's a interesting story, there.
https://onepiece.fandom.com/wiki/Squard
Attach! AKA the guy who takes the bounty poster pictures for the Marines. (AKA Jimmy Scoops.) Mugiwara No Goofy has kind of taken the character and ran with him. And there actually is a lot of story potential for a character like this. A ordinary guy who has to sneak around a bunch of monsters constantly.
https://onepiece.fandom.com/wiki/Attach
Shu! AKA the Marine with the Rust Fruit. With a Devil Fruit like that, it feels like he really should have been a bigger deal, and gotten used more often. But, he's only appeared once (in canon)!
https://onepiece.fandom.com/wiki/Shu
Enel's Divine Soldiers! AKA The Birkan Goat-People. These guys are completely different than any other Skypiean we've encountered. Why are they part goats? Are they just Sky-Minks? They're just so random and weird that I want to know more about them.
https://onepiece.fandom.com/wiki/Divine_Soldiers
Ahiru! AKA the Revolutionary with the metal arm! I don't know, I just think she looks interesting. And maybe she'll eventually play a bigger role down the line. Who knows? I feel like there's a story behind that metal arm.
https://onepiece.fandom.com/wiki/Ahiru
The guy who asked Roger about the One Piece at his execution at Loguetown. AKA Ask D Question. We don't even have a name for this guy. It'd just be nice to find out who he was and what he went on to do. Again, he plays a big role in a iconic scene...but then he disappears.
You could also do something with the Marines who actually executed Gol D Roger. Those guys are just faceless grunts, but they feel like they should be pretty famous in this world.
The creative team behind the Sora Warrior of the Sea manga. I feel like you could get a story out of people trying to find creative ways to make a propaganda comic.
King Seki and/or Princess Komane of Lulusia. I just want to know how vampires work or don't work in this world.
Nojiko. This one's right on the edge. But, I can't help but feel like Nami's sister knows more than she lets on. I feel like she's hiding something.
Kong, Ochoku/Wang Zhi, Milky the reindeer Mink, The Yeti Cool Brothers, Mozu and Kiwi, The Charlotte Decuplets, Belo Betty, The Numbers, Momousagi, and Doll all get honorable mentions. I don't know what you'd do with them, but I wouldn't mind learning more about them.
