Brian Tyler. He's scoring everything else Zimmer isn't.
But seriously, either somebody the director has worked for (we don't know the director yet), or maybe it will be like Williams -- the name is so attached to the film, the new director will just seek Conti out (if he even wants to do it or is free to do it).
"The film will focus on a man following in the footsteps of his grandfather, Apollo Creed, and getting a mentor in the now-retired Rocky Balboa."
What might have been a better concept: an historical, decades-spanning period-piece in which the young Apollo Creed reaches manhood and comes up through the boxing ranks. But there's no role for (elderly) Rocky in that saga, so Stallone probably wouldn't like it.
My bet is on Brian Tyler. Rambo, Expendables, he's got the current track record with Stallone. I imagine Tyler will try to incorporate some of Conti's style into the score, but my expectations for this film and score will probably not be very high...
If Stallone is producing, I would say it looks good for Brian Tyler.
Well it may be true but there are a slight chance that it will be someone that the director wants. The Stallone written (and produced) movie Homefront starring Jason Statham was not scored by Brian Tyler (or Bill Conti for that matter). It was scored by the directors usual choice of Mark Isham so with that movie in mind it could be someone else.