Short Answer: Mickey's story
The Rocky franchise has had a very successful run with the Creed series. The idea was that the same way Rocky had been mentored by Apollo, he would now mentor Apollo's son, Adonis.
But another spin-off is also possible. While the Creed series spins the story into the future (Part 3 of the Creed series is slated for 2023), the mentorship chain also can be traced backward, into the past.
Rocky's beloved mentor, Mickey Goldmill-- who we see pass away in Rocky III-- also must have a story. I see the template being the very successful Ip Man series, about the man who mentored Bruce Lee (Ip man also had 4 parts and a spin-off of its own).
There are obvious parallels between boxing movies and martial arts movies-- the Karate Kid storyline was clearly inspired by the Rocky formula, and look how well it did, how long it lasted. It also spent some time delving into the past of the mentor figure, Mr. Miyagi.
One great thing about the Ip Man movies is that they showed life in the past. The Mickey Goldmill series could do the same-- showing the early stages of organized boxing in America, when the boxer's ethnic identity was a key part of their marketing. Even Rocky was "The Italian Stallion" in his day, and that kind of branding has a long history.
And yes, we would see-- in the background, and we learned about Mickey-- how the Irish, Italian, Hispanic (to use the term from then) and other ethnic groups that boxed their way out of the slums, as well as the rise of the Black boxer.
But, Mickey's character being Jewish, it would be especially heartening to see the Jewish boxing greats of the past, like Benny Leonard (even if they were fictionalized versions, like in Dreamgirls). Also Max Baer-- so awfully handled in Cinderella Man-- could finally find an honest, honorable depiction.
Setting things in the 1940s and '50s is a lot of fun for the set and costume designers, the music director, and other creatives in the films. And Jewish stories set in the 1950s, like Mrs. Maisel, have a built-in audience.
So let's plan a trilogy about Mickey Goldmill, and the events in his life that led him to fund and mentor Rocky Balboa. Figure out how old he in in the first Rocky movie, then work backward. Show us how he got into boxing, why he ended up a trainer, where he learned the techniques-- like chasing a chicken!-- that we see him use in the first three Rocky movies.
The only tricky part would be the title. Calling the movie Mickey would make people think of Mickey Mouse. For the Creed series, they used Apollo's (and Adonis') last name, but it was also handily a word. Not enough people know that Mickey's last name was Goldmill, so that might not work.
Also, Mickey last appears in a flashback scene in Rocky V, a movie that came out in 1990. There would have to be a marketing campaign reminding the public of what a central figure he was in the Rocky series.
So a Mickey Goldmill series would have an uphill battle to fight. But that's something that's baked into the whole Rocky idea anyway.