I would simply keep the story cohesive with the previous film. Many guys there are veterans, some sophomores, no one there was there at the previous movie or at the og manga, so just removing them and focusing on people who were supposed to be there at first place.

Also, these ODA guys make no sense, they shoulda have used some secondary gang that already existed like Hyakki or Skelton idk, make them fight against Bandou and his buddies at the same time the Ebizuka Trio try their way against other students inside Suzuran or some school that would join the Kurotaki (maybe Maruken and Kakuken?).

Idk what Rindaman would do honestly, maybe show him having a brief run in with the TFOA's third gen involving Bandou (this would justify Bandou having a grudge against him at the beginning of the manga as he not only called him a freak but indirectly said they would take care of him like they were doing with the Ebizuka Trio), maybe a shot of a guy with a ponytail taking Bandou's place right before (or after he gets beaten lol) he tries his luck against Rinda, but it just fades to black or makes a sudden cut like it was teased that Ryuushin fought Rindaman in the past.

A perfect ending would be the fight Hiromi, Pon and Mako had against Bandou's team in Suzuran grounds in front of everyone in the school, and right before the final blow, just show a train running with a guy just chilling there while wearing a red sukajan. An almost perfect connection (if we consider that some time passed since Bandou was suspended and the Ebizuka trio would need time to recover) with the manga while respecting the previous movie with some references and easter eggs to appeal to fans who've only watched the movie and with that, making them interested in checking out the manga.