The Aluminum Falcon said:
A Star Wars feature, live-action film without John Williams? Unthinkable.
Well, ROTS had limited Williams participation because of how it was edited. It was either ROTS or AOTC that Williams never got a good final cut to work with/score. Still, no participation whatsoever would be bad. Hopefully, they don't jettison his work altogether and use... DANNY ELFMAN!
It was AOTC, I heard Williams didn't contribute much because he was pissed at how butchered his score for TPM was in its final cut.
Meanwhile, I doubt they'd flat out ignore his contributions. They even use some of his themes in Clone Wars. If a TV series can, feature films can. I just wonder how much they'll pay attention, it could go two ways (assuming Williams doesn't come back):
1. They simply put quotes of the Force theme here and there, but otherwise have new themes everywhere (like Potter 4-8 did with Hedwig's Theme).
2. They actually look at all of his themes for all the characters and use them ("Luke and Leia", the Droids' theme, etc.).
Unfortunately, the first option is the most likely, more or less.