I thought of a Sing 3 idea that I really like.
It takes place after Sing 2,maybe a few weeks-a few months after. Buster still has trauma (rightfully so), but he acts like everything's fine around the gang. Eddie comes back, and Buster is really happy, but he doesn't know about Buster's trauma or about what happened. Throughout the movie, Eddie gets increasingly jealous of Buster's friends but tries to hide it. When Buster tells his friends that they can start calling him Buster, Eddie gets convinced that Buster doesn't care about him anymore. At some point in the movie, Johnny gives Buster a newspaper with the headline that Crystal has broken out and that he's furious and will get revenge on his enemy, the one who put him in jail, Buster. Buster freaks out inside, but he again hides it and brushes it off. Then, later in the movie, Eddie talks to Buster about his jealousy and Buster reassures him that he's always his number 1. Then Eddie asks about why he looked so upset reading that newspaper and Buster just breaks. I don't know anything else. Obviously there'd have to be, y'know, singing in it, but I don't know what it'd be. Maybe Buster can sing about not being afraid of Crystal anymore.