Which one that we WANT to see, or which one that we think is most LIKELY?
For me, I'd LOVE to get a follow up to Enel's cover story after all these years. Or to see Perona and Moria's adventures, after escaping Hachinosu. Or Big Mom and Kaido's adventures in the "Underworld", if they're still alive. Or Vivi and Big News Morgans finally arriving at wherever they're trying to go. Or the Revolutionary's fight against Mariejois. Or seeing the mission that Shamrock was sent off to do while Elbaph was happening. Or Cross Guild's antics in freeing various former villains from prison so they can swell their ranks with the likes of Doflamingo or Weevil. Or following up with Smoker and his mystery box. Or the Vegapunks arriving at Weatheria or a Sky Island, and maybe meeting up with Urouge. Or Stussy finding herself a new job. Or Law, Bartolomeo, and Kidd all running into each other and forming a new crew together, the Soggy Bottom Boys. Or the rest of the Strawhat Grand Fleet trying to meet up with Luffy. Or just another "Decks of the World" series, where we get updates on everyone around the world.
But, Oda does like these cover stories to feature characters who are a little more obscure. And he doesn't like to repeat himself, too much. So, it would probably feature someone who hasn't gotten a cover story before.
Kuro's adventures trying to find a peaceful place to live, with him ending up on Hachinosu with Gin and Don Krieg?
Doctor Kureha journeying around the world on "vacation" before she makes herself the new "idol" of Hachinosu, taking Shakky's old job?
The Baratie getting hired to cater the Celestial Dragon's next Rabbit Hunt?
Yosaku and Johnny trying to hunt down Ax-Hand Morgans for his bounty?
Water 7 getting turned into a true floating island, like Thriller Bark, maybe with Moria and Perona's help?
Mont Blanc Cricket discovering the fabled Emerald City on Nakrowa?
Rebecca and the Tonttatas turning the abandoned SMILE factory into a theme park for the Punk Hazard Kids, maybe giving Sugar a chance to redeem herself?
Carrot getting tossed onto a Sky Island while Zunesha is having himself a snack, and she thinks she's dead, but she gets to meet up with Conis and Aisa?
Arlong and other Fishman prisoners being forced to excavate some undersea ruins or a sunken ship for the World Government?
Shalria joining the God's Knights and getting some special training, so she can learn the Abyss magic?
Kokoro expands the Sea Train all the way to Fishman Island, so she gets to reconnect with some old mermaid friends of hers?
The escaping Egghead scientists giving free "upgrades" to every island they visit, and then accidentally reawakening another piece of Super Ancient Tech, like the lab we saw on Elbaph or maybe another giant robot like Emet?
There are SO MANY characters we could check back in on, it's kind of crazy to think about.
...Just as long as we don't get Wano Act 5. Anything but that, I'm begging you, Oda!