I don't remember this being discussed, there was apparently an official statement that not every character from PW3 is returning as playable.
If we did a process of elimination and made some assumptions, lets say every Strawhat, Shichibukai, Yonko, Marine Admiral and arc villain is guaranteed to stay.
Leaves us Garp, Ivankof, Tashigi, Ace, Perona, Law. I think Garp, Ivankof, Tashigi and Perona are probably low priority. I'm not even convinced all the characters I excluded are even that safe. For instance, if the goal is to get closer to a faithful adaptation of the story than previous entries, then the antagonists have less value being playable since the story is told from the "hero" perspective.