I always felt that Oda would be open to others expanding on his material, either him in an advisory role or even people expanding on it even long after the fact. It might sound absurd until one takes into account things like One Piece fan letter. I dunno, there's prob lots fo stuff Oda will never touch on that other writers will always be happy to expand on if they're ever given the chance.
Just adding onto this because man, there's tons of things that Oda has already done in terms of expansion.
Want more Ace? One pIece Novel A which was then later adapted by Boichi. Also on that note, I don't think it's a coincidence that Rocks has the design elements of a Dr Stone character, from multicolored paper-like hair to a distinct crack-like scar.
Want more Law? One Piece Novel Law
Want more cute side stories with the girls? One Piece Novel Heroines, which charted very well in the light novel market.
Oda has the benefit of the Shonen Jump novelization architecture, and he has all the resources to outline and then hand off and then profit.
Already, One Piece has a very expanded universe from just the supervised works. Now, will One Piece ever have a novel on par with Bleach's Can't Fear Your Own World? It really depends on if Jump can bring Ryogo Narita or Kazuma Kamachi in. They're probably the only authors in the light novel scene that could exploit the maximum narrative potential that the One Piece setting has.
One more thing on this point is that the enemy of the expanded universe is Oda's own lack of timeline management. Which, interestingly, is a problem that's systemic in Shonen Jump. But yeah, because of how tight everything is in One Piece's timeline, the expanded materials can't breathe as easily as the Hero Academia movies can. Pre Timeskip all happening over the course of a few months, and being less than a year is the biggest hurdle is one of the most obvious effects of the timeline mismanagement.
