Reports say that Apple will call its iPhone X followup the "iPhone XS", with the larger 2018 iPhone model being called the the "iPhone XS Max" instead of using the standard "iPhone ... Plus" moniker.
The "S" in the name implies that 2018 may be an "S" cycle year for its iPhones. Typically these are the years when Apple's new iPhones get under-the-hood updates (like upgraded processors and features) instead of major design changes (think iPhone 6 versus iPhone 6S).
A recent report from Bloomberg corroborates by saying that Apple "is likely" to use the "iPhone XS" and "iPhone XS Max" names, citing "people familiar with Apple's internal deliberations."
And the third of Apple's rumored 2018 iPhones, the affordable 6.1-inch LCD model? The report says that Apple "has considered" calling that one the "iPhone XR." That hedging phrase leaves the possibility wide open that the iPhone could be called something else.