Stussy's side of the story would be kind of fun. But, I think we'll probably get something else from Oda.
Maybe something with the Vegapunk's journey through the Sky Islands and adventures in Weatheria?
Neo MADS trying to catch up to the main story and finding out that Vegapunk is (sorta) dead? Which could directly tie into the previous idea.
Maybe following up with Law and Bepo and finding out where they went.
Dragon and the Revolutionaries assault on Mariejois would be a good thing to cover.
Garling's training regiment to become a full member of the Gorosei?
Vivi's varioius adventures with Big News Morgans?
A completely random story about Aisa and Conis from Skypeia?
Captain Kuro having misadventures and somehow ending up on Hachinosu with Gin and Don Kreig. Maybe he gets chased around by Yosaku and Johnny. It's been awhile since we've seen ANY of those guys.
The Big Mom Pirates looking everywhere for Big Mom.
Doflamingo breaking out of prison. Maybe getting Arlong and a few others to join him?
I mean, it could basically be ANYTHING. I was not personally expecting one of the cover stories to basically be Wano Act 4, after all. Oda could pull ANY random character out and do a cover story about them for fun.