A Nika and/or Joyboy sidestory would be the obvious choices. Personally, actually getting to see the fall of the First World seems more appealing to me. It'd end up being a tragedy, but it'd be interesting to see this advanced society destroy itself. Seeing the rise of the Gods. There's a lot of stuff you could flesh out here.
Enel's further adventures on the Moon could be fun. Especially if Enel starts visiting other planets and trying to conquer those. Starting his own intergalactic empire.
Koby's story throughout the manga always struck as a possible parallel story. There's a lot of stuff we've skipped over with Koby. I could see a spin-off with him as the lead lasting a few volumes, anyway. Showing the Marines in a more positive light, and featuring more villainous pirates than we usually see in the main story. We could even learn a bit more about SWORD that way.
Sabo and the Revolutionaries could probably also support their own manga. Their plans and goals are different enough from the Strawhats that their adventures could be different enough to be entertaining in a different way.
Similarly, seeing the whole story from Blackbeard's point of view could be kind of interesting. It'd have to happen after the manga is completely over, though, to avoid spoiling what he's planning in the main manga.
Toki's story specifically could be kind of interesting. How many other time periods did she just to before she reached Oden's time? Could she have actually survived her "death", and journeyed into the future? You could do something interesting with a time-travelling main character, who can only ever move forwards and never go back.
A story focusing on a Fishman or Mermaid would be interesting. We've had a lot of adventures on the surface of the world, but we've only really visited Fishman Island under the ocean. There's an entire world and society under there, just waiting for someone to do something with it. What kind of legendary places and adventures await in the other 90% of the planet? What are the other major Fishman cities? What kind of mysterious ruins from the First World are under the ocean, as popular Fishman Tourist Traps? It'd be a good chance to tease some things that are down there from the Void Century or First World. Maybe one of the side-characters could actually be a Sea King that we get to know. Maybe we could get some hints about that centuries-long plan they had involving Shirahoshi and Poseidon.
Crocodile's history and his formation of Baroque Works could probably support a side story. It feels like there could be a lot of meat there. A rookie rising up the ranks to form a secret criminal organization has some story potential.
The Many Romances of Big Mom could probably be it's own manga. Every chapter features her capturing a different future husband. Different arcs where she's husband-hunting someone specific. There's stuff you could do here. It'd be...kinda dark. But, it could also be played for some dark humor. There's potential here. Messed up, twisted potential, but potential all the same.
Depending on what Oda's plans actually are, a Mihawk sidestory could be interesting. I don't know if it would last more than one volume or so, but it could probably be fun. Seeing his rise to attain the title of World's Strongest Swordsman. Finally finding out what that actually means, who decides this kind of thing, and who had the title before Mihawk. Maybe even learning some more Named Blade lore.
Speaking of which, a Tashigi sidestory has a lot of potential. A story about a girl travelling around the world trying to collect a bunch of legendary swords? That's actually a pretty solid premise for it's own manga. Goodness knows Oda's never going to actually give us names for all the Named Blades in the story, except maybe the very top grade ones. So, having all that stuff get fleshed out in it's own side-story could be fun.
Seeing the rise of a Celestial Dragon into becoming a God's Knight would be interesting. At least that way, we could see the story more from the Celestial Dragon's point of view and see how they see the world. Learn more about their culture. I'm not sure if featuring a known God's Knight would be better like Qilingham, or if a brand-new character would be best, who could end up coming into the main story later as a cameo in the main manga.
Maybe a Gale-Ra Warhammer Company sidestory could be interesting? I mean, it seems like they're pretty key to the story, I guess. A sidestory spinoff could help flesh these guys out. It sounds like they had a lot of adventures. It could be a fun time.
Maybe instead of following a certain character, we could follow a object, instead? Like trace the history of the Straw Hat through all it's various owners throughout the centuries. Or follow a certain blade as it gets passed down from generation to generation. Like maybe show how Enma ended up getting "cursed" in the first place or something. It could be a fun way to spotlight various random moments in this world's history.
King/Alber's journey might be a interesting one to follow. How did his parents die? Where did he come from? Again, I feel like it wouldn't be a long story. But, maybe for a one-shot volume.
Moria's adventures trying to steal and/or construct Thriller Bark? I imagine it'd be more comedy-focused. But, this has always struck me as a odd engineering marvel. Showing us how Moria actually managed to take a part of a island (possibly part of God Valley) and turn it into a ship just feels like it's ripe for some sort of shenanigans and misadventures and setbacks.
Foxy's misadventures. Just arc after arc of Davy Back Fights. Showing how he wins his crewmates and loses them all again. The story of a compulsive gambler who just can't stop. There's more story potential here than a lot of fans want to think. Plus, it'd be a chance to spotlight some random islands that we might not see otherwise.
I really don't want this. But, a spinoff set in Wano would make sense. Either featuring Ryuma or Yamato. Oda clearly can't seem to let this island go, for some reason. So, it would make a lot of sense if Oda wanted to do a spinoff set entirely in Wano. I'm not sure what the point would be, but I could see Oda wanting it to happen. If only because it seems like he put a lot of effort into designing and world-building for the region.
How about a manga that shows us Dorry and Brogy's fight? ALL of it. All 100 years of their fight. Non-stop. You'd have to get creative to keep it from getting too repetitive, but that's where the fun would come in. Trying different tactics and dealing with different random stuff happening around them.
I could see an anthology story focusing on the Royals from the Reverie. Give us one-shot stories fleshing out the kingdoms and people that King Tacos or Ham Burger rule over. Help flesh out these random islands we never get to see in the main story. Tell us more about their culture, so that we can actually care about them, outside of their designs.
A Sora Warrior of the Sea manga. Combining parts of a "real" volume of the in-world manga, and a story about the manga team who has to make this propaganda comic to the World Government's standards. Having to deal with the Gorosei giving "notes" and feedback about what they want to see in the story, as their "editors" and owners. Give us a Gundam Sousei, or Bakuman, or Nozaki-Kun set in the One Piece World. Again, it could be fun.
Mugiwara no Goofy's already mined this for it's comedy potential. But, a side story focusing on Attach, the photographer for the Newspaper, trying to get updated Wanted Poster photos of various characters has a lot of potential. Trying to sneak around the sidelines of major stories just to snap a quick photo and all the misadventures he'd get up to trying to get a good shot. Honestly, even a World News sidestory has potential. Dealing with Big News Morgans' various whims. Trying to get a better job by taking outrageous chances to get a interview. A Newspaper-centered story has potential.
And, yes, I still want "One Piece...of Our Hearts"! Give us a anthology of one shots looking at how various couples in the One Piece world got together. Just give them to some shojo romance mangaka and let them get really sappy with it. Roger and Rouge! Rayleigh and Shakky! Scopper and Ripley! Big Mom and her Three-Eye Tribe Husband! The main story doesn't really do romance, so give us ALL the romance in a spinoff story. The shippers demand it!
