Well, Roman numerals past X would be strange. I also don’t think Pro is likely since next year, or 2020 at the latest, OLED will be mainstreamed like FaceID was this year. At that point I think they’ll drop back to two new models per year, with size being the most obvious difference again.
Then Pro wouldn’t make any more sense than it would have in a pre-X year. iPhone 7/7 Pro would never have made sense, they’re just too similar, and there’s just a $100 price difference.
Personally I’ve always thought X.2/X.2 Plus (like MacOS: “ten point two” or “ten dot two”) makes the most sense; it lets them reset the numeral so you won’t have 11, 12, 13, etc. going forward. It would just be weird to have iPhone 21 10 years from now; at some point it had to change.
As the iPhone 8/8 Plus follow-on, iPhone 9 (or 9 Plus) makes the most sense to me for the LCD, and makes it obvious that it’s still “less than” the X models whether 3017 or 2018. If they introduce an updated LCD model next year instead of going all-OLED, it could be the 9S.
But the idea that Apple hadn’t already considered this year’s naming even before they introduced the X last year isn’t believable. They already had a roadmap in place and knew what the iPhones this year (and beyond) would be called. It is a challenge and it’ll be interesting to see what they decided.