Other Possibilities
There are other, more vague iOS 13 rumors out there. Many of them were originally planned for iOS 12, but were delayed in favor of the aforementioned stability and performance improvements.
For one, Apple could update its Camera app — possibly adding more in-depth photography controls that the app has always lacked. There are also a slew of improvements to “core applications,” such as Mail, that were originally rumored for iOS 12.