The launch of iOS 8 hasn't been without its hiccups, and it seems that Apple is putting a lot of work into ensuring that its successor — we'll call it iOS 9 (seems like a safe bet) — irons out those kinks. iOS 8 adoption has lagged behind previous major version numbers of iOS, though it's worth noting that it is still doing quite well in comparison to the competition.
Regardless, the focus for iOS 9 is said to be on the inner workings of iOS. Says 9to5Mac:
For 2015, iOS 9 is going to include a collection of under-the-hood improvements. Sources tell us that iOS 9 engineers are putting a "huge" focus on fixing bugs, maintaining stability, and boosting performance for the new operating system, rather than solely focusing on delivering major new feature additions. Apple will also continue to make efforts to keep the size of the OS and updates manageable, especially for the many millions of iOS device owners with 16GB devices.
Of course, bug fixes and improved performance aren't the only things that will find their way into iOS 9. If that's all it was, it would be iOS 8.4 or something similar. Bug fixes and other enhancements are standard fare for any system update, although it sounds as if the recent criticism about Apple's software quality — normally one of their strongest suits — has been taken to heart.
There will be new major features in iOS 9, naturally, just like the complete redesign and Touch ID of iOS 7 or the addition of Apple Pay and support for sharing and action extensions in iOS 8. What those major new features might be, we don't yet know, but we'd keep an eye on WWDC 2015 sometime this summer for more info.