I take and edit a lot of photos with my iPhone and the built in Photos app is great. But I wish there was more sliders and options to play with such as sharpening and vignetting that are already present in the Photos app on the OS X. So I would be delighted to see more of these tools on the phone.
It would also be amazing to see iOS wide “dark mode” that can be turned on automatically in the evening and night hours. There are apps and sites including our own Cake that would greatly benefit from the night reading experience.