This question relates very personally to how you felt about the book. Only you can say how would have resolved it differently. That being said, consider some of the following questions, which might help you consider what to include in a chapter 12:
Do Mark and Bryon ever "make up"?
Does Bryon ever get over what happened? Does he ever move on?
What helps Bryon get over his sense of loss?
Will Bryon and Cathy ever get back together?
What will Bryon do with his life now that he is an adult?
Will Mark ever seek forgiveness for what he did?