Based on some of the recent comments I've read I figured I'd add my opinions on them as well.
We already had a former bad guy return with his son in Creed II (Viktor Drago, Ivan Drago's son). I don't feel we need to repeat that with Clubber Lang's Son.... The reason Drago's son was important and a "full circle" type deal is because Ivan Drago killed Apollo. I just don't see the need to continue the previous bad guys return with their son type stories. Give a new original "villain" for the film and I use the term "villain" loosely, perhaps I should say "opponent".
Also, I agree, Rocky's story is complete. It was completed in Creed II with a "full circle" approach with the Rocky vs Creed vs Drago saga coming to a final close. His farewell may not have been what everyone wanted but it was fitting in my book. He exits basically saying "This is your story now".
If Rocky was to return at all I think it should be a small non-speaking cameo. I don't want Rocky to die like Mickey. Enough call backs in the previous two Creed films. If anything I'd personally like to see something like after Adonis wins whatever huge fight he is part of and he looks out in the audience at the end of the film looking for his wife he sees not only her but also Rocky, Rocky's son and his grand children watching on in support. Just so we know he is there and still doing well as Rocky deserves that closure with his remaining family after the various hard times he has dealt with over the course of the franchise.