Since the iPhone 6 and 6 Plus, the increased screen size has caused new issues for single hand navigation around the OS that I notice on a daily basis.
The worst impact is the go back edge pan that pops a view controller off the stack, which I think was the best way to navigate back on iPhone 5(s). The more you can do at the lower half of the screen, the better.
But a still un-tried way to improve navigation could be found in the home button, or technically the fingerprint reader. Imagine you could swipe across the button to navigate backwards, or scroll up and down - that would be amazing.
How about detecting force of the fingerprint to allow for more granular control in games?
It's coming, perhaps not in the next iteration but a patent application seems to verify it will arrive at some point.