A side story of OP without the SH is cool, but there are few characters that could feel the gap of nothing have Luffy, Shanks for sure is one of them.
Probably something about young Shanks and his crew travel from East Blue after Roger death.
Dont get your expectations too crazy, wont be a movie about events too important for in-story plot, but we could get some cool things like Young Shanks and Young Mihawk duel, and even the story of the scar, expecially if by mid of 2022 we got more Shanks in the manga, because the film would be a good way to hype the fandom more about Shanks time to shine in the main story finally comming.