Adonis feels guilt. Slips into a depressive alcoholic state not finding the answer in having beat Drago's son. Some people view him as a murderer. One guy drunkenly harasses Creed and says something that triggers him "Did killing someone bring your father back?" He loses it and assaults the guy. This incident is caught on cell phone cameras and goes viral. His hands are registered so the fault is placed with him. He does a prison bid.
Rocky and Bianca are appealing to the courts during his time in prison. While inside he meets an older and wiser Clubber Lang. Clubber helps him to control his anger, and his demons. He coaches him as he takes part in the prison boxing league. Clubber shows Adonis a new style similarly to how Apollo helped Rocky. This allows him to beat Pretty Ricky Conlan who is the league's champion, who Adonis lost to in the first movie. While this is going on fans are petitioning the government for Adonis' release and they win his appeal stating he acted in self defense.
Having been stripped of the title Adonis works a few fights and becomes the number one contender for the new champion (who I would choose to be played by Deontay Wilder). Drama unfolds, and the match is set. Adonis fights tooth and nail with him using the style Clubber had shown him and being coached by Rocky. He wins the fight.
In the post fight ring interview he catches eyes with the guy who he had fought that resulted in him being sentenced to prison. He pauses and then says how this fight was about redemption, for his career, and for him as a person. The two lock eyes and the man silently forgives him for his part and nods. The movie ends with everyone cheering as Rocky movies typically do.
This is what I've got, could make some adjustments but it's a start lol.