My vote would go to James Newton Howard who has a classical background. Perhaps Thomas Newman also.
Interesting suggestion, especially since Williams and the Newmans go way back, and Williams has spoken highly of Thomas Newman.
I think if you put so much restraints on the new composer(s) so that he or she has to almost “disregard” their own style to make it sound the way Star Wars has “always” sounded, you have already lost.
I’m not saying composers should disregard their own style, but rather stay away from stylistic traits which definitely don’t belong. For example, I don’t think Desplat’s mechanical, repetitive patterns would fit well in a Star Wars score.