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. Even John Williams himself took the music in a lot of new directions with the prequel trilogy (and did some of the best work of his career, IMHO). I’m sure he wouldn’t want his followers to imitate his style or styles, more than doing their own thing.
That’s why I would prefer someone other than Giacchino, who would maybe stick too much to the pastiche thing.
That said, I don’t exactly want a Daft Punk or a Trent Reznor Star Wars either. Neither do I want a Hans Zimmer Star Wars.
Someone who doesn’t stray too far away from the classical way, with an orchestra and with themes, but still has their own voice to add to it, instead of just imitating Williams.