Like someone else said, the low hanging fruits (3rd party apps, Multitasking, Copy-Paste, Notifications), are all gone by this point. iOS is starting to mature and smartphone OSes will drift into the same cycle of subtle refinements desktop operating systems have been in for the past 10-12 years.
I suspect the next big thing for iOS could involve interoperability between Apps. So far apps live each in their own little sandbox, oblivious to one another. Working between different apps becomes tedious fast because all tasks must be done serially.
There's still some things they could do: They could allow third party apps to hook deeper into the OS; they could allow us to pick out default apps for different tasks. They could give us "full" multitasking for 3rd party Apps (current multitasking is limited as it's based on a set of APIs accessible by 3rd party apps. Default apps like Safari and Mail do have true multitasking).
But I doubt that'll happen because that would go against the design tenets that Apple established when they created the iPhone. Or maybe they'll surprise us. Who knows.
I think that the iPad would benefit if Apple stopped treating it like a gigantic iPhone. There's absolutely no reason for apps like Messages, Twitter, Facebook messenger, and even Mail to run full screen, all the time. Such a waste.