I would love to see Johto as well, it would make the game feel more complete imo. Maybe they could add it, and modify the e4 so that Johto has it's own, completely new set of the e4. Instead of Lance showing up at key parts, it's Johto's own champion. Sure that part might not be nostalgic then, but at least the rest of Johto would, plus Johto having it's very own e4 and not having it be shared with Kanto's would be pretty cool, I think.
**Edit** Could make Eusine the champion, Janine an e4 member (instead of Koga), keep Will and Karen... And to replace Bruno, maybe the cut master (fire e4) from Azalea town? Or the master sage from Bellsprout Tower (grass e4)