Imo Oda would not start another long serie after finishing One Piece, so I'd say that he could do groups of volumes to complete the story, for exemple:
- North Blue: showing Law, Drake and Hawkins path before coming to Sabaody and how they earned their bounties
- South Blue: same thing of North Blue, but with Kidd, Killer and Bonney
Each blue could have 50 chapters each
Same format could be used for OG Warlords like Moria, Croco, Doffy... hell I'd love something like that