The Fighter Part II ?
A while back I remember reading an article where Mark Wahlberg said there would be a second Fighter film chronicling the rest of Mickey Ward's career but I haven't heard anything as of late. After the events depicted in The Fighter, Ward went on to have a legendary fight trilogy against the late Arturo Gatti. Ward won the first fight and lost the next two. All three fights were total wars. Ward and Gatti also bonded and became good friends.
I think hardcore boxing fans would like a Fighter Part II if done well. But it might be a tough sell with general movie goers.