Rumours suggest the iPhone X design could get two more sizes, mini and Plus, with upgrades to the internals of the X itself. Others suggest there could be cheaper alternatives that keep the design.
Either way, expect more than one iPhone this September. Apple had to play catch up with the iPhone X, and it won't want to do that again. Likely with an A12 processor, better battery tech, iOS 12 and faster charging, this is going to be the best iPhone