Every year, Apple put a significant upgrade on the iPhone. People that keep saying the S is the same phone with "minor" updates as the number cycle basically only judge by the physical appearance, or don't have much technical background. If one looks at the improvements Apple did to the SoC, every step is significant.
There are still a lot of improvements to be made for a phone, even on the "basic" features. Just examples:
1. Touch screen -> improve response and refresh rate (eg. iPad Pro)
2. Camera -> improve low light, image stabilisation algorithm, larger aperture, improves ISP
New features:
1. Waterproofing: should be a standard feature by now considering we use our mobile devices almost 24/7, and weather doesn't wait
2. New battery technology: anything to improve the length of time we can use our device untethered.
The beauty of the Apple ecosystem is that the strides in software goes hand in hand with the hardware, usually. That's why sometimes we see a feature coming out "late" vs the other platforms, yet when Apple did it, it felt "right." Other OEMs that rely on off the shelve components cannot have the hand-in-hand approach as easily.