Powell's X3 score is underrated and easily the highlight of that movie, imo. I wouldn't had mind if he had returned to score this, however, he already tackled an X-Men movie that somewhat dealt with Dark Phoenix, albeit poorly.
Marco Beltrami would've been another worthy composer because he's had experience scoring some Fox/Marvel movies The Wolverine, Fant4stic, and Logan. The latter two were produced by Kinberg.
Like I mentioned earlier I think Zimmer will farm out his score to other composers (possibly Lorne Balfe, Benjamin Wallfisch, Steve Mazzaro, Andrew Kawczynski, etc.). Zimmer will most likely come up with the themes, motifs, and certain cues, while his collaborators put the rest of the score together utilizing Zimmer's themes/motifs. Hopefully he'll keep Junkie XL away from this.