Fun pattern to consider:
Forstall Era
iPhone OS 1: Introduction
iPhone OS 2: App Store
iPhone OS 3: Various improvements
iPhone OS 4 / iOS 4: Multitasking, Facetime, & iBooks
iOS 5: Notification Center & iCloud / PC Free
iOS 6: Passbook & Various improvements
Ive-Federeghi Era
iOS 7: Reintroduction of how iOS looks and works
iOS 8: Apple Pay, Health, Continuity
iOS 9 (Speculation): Various improvements
iOS X (Speculation): Large scale changes to go with the X
So then, iOS starts with an innovative way to interact with it, adds new purchasing aspect every even number version, and the third round has a greater emphasis on performance and touching up little overlooked things
Forstall Era
iPhone OS 1: Introduction
iPhone OS 2: App Store
iPhone OS 3: Various improvements
iPhone OS 4 / iOS 4: Multitasking, Facetime, & iBooks
iOS 5: Notification Center & iCloud / PC Free
iOS 6: Passbook & Various improvements
Ive-Federeghi Era
iOS 7: Reintroduction of how iOS looks and works
iOS 8: Apple Pay, Health, Continuity
iOS 9 (Speculation): Various improvements
iOS X (Speculation): Large scale changes to go with the X
So then, iOS starts with an innovative way to interact with it, adds new purchasing aspect every even number version, and the third round has a greater emphasis on performance and touching up little overlooked things