His work on the first Harry Potter films was quite good in my opinion. He managed to make some very memorable motifs. The great thing about bringing back the Star Wars characters like Luke, Han, Leia, and Lando, he can bring back their motifs.
Each character had their own, it's easy to work around a melody you've already made.
Joel McNeely made a very good soundtrack for Shadows Of The Empire. It used the familiar motifs, but it veered of in it's own direction, while remaining William's esque. I'm shocked Fan Editors don't use that soundtrack for prequel edits.
They just need to tell John Williams to emulate Holst, Mahler, and Stravinsky. We'll get an amazing soundtrack. That's essentially the way the 77 soundtrack was made.