Nothing wrong with Xs plus IMO. I mean it's not great, but consumers already recognize the nomenclature and it doesn't have to be explained, why tank a system that's working? So this year it would be Xs, Xs plus and the lower range iPhone would could be iPhone 9. Although that would create problems for next year I suppose as they couldn't name it the iPhone 10. The more I think about it, the more it's possible maybe they are using both plus and max. So the low range iPhone would be plus to signify the larger screen, and the larger OLED iPhone would be max to signify the largest screen.
Or maybe they give it some random name like iPhone SE and call it a day, who knows.