A Creed III without Ricky Balboa could continue to do that or it could go in an entirely different direction, giving Adonis Creed his own sound, which the movies have largely already done.
The most iconic pieces of Bill Conti’s original score certainly appeared in the Creed movies, but the music was used sparingly.
It will be interesting to see how the score of a “Rocky” movie changes if Rocky himself isn’t even in it.