iOS

Voice Assistant API: Adding your own voice assistant technology isn't terribly difficult, albeit expensive. By adding system-level integration, apps can provide a significantly richer experience for Siri-users. The first level of integration could be simple queries that Siri can pull from CoreSpotlight, which was introduced last year with iOS 9. If your app uses this capability, you're already good to go for Siri queryable services. The next level of integration is actual transactions. By creating a separate app extension, Siri would be able to use part of your application to perform transactional work, like ordering a pizza, using a third party weather app, etc.

Public CarPlay API: Apple's current program is by invitation only. It'd be great to be able to offer CarPlay capabilities augmented with Siri. I recently got an aftermarket CarPlay unit for my car and I love it.

iCloud offline sync + cross platform support: If you're not developing exclusively for Apple, iCloud is pretty much off limits. Even though they have a JavaScript API, they really need to up the game on their services to not only support other platforms, but support things like offline sync to compete with other services, such as Dropbox and Firebase.

More public API for UIAlertController: The illusive part of UIAlertController was being able to add custom views to alerts. If you wanted to add anything custom you had to roll your own complete alert and transitions to boot. Also, the action sheet style of alerts has a fair bit of private API for adding custom images to buttons, as well as adding checkmarks. I'd like to see a lot of those things opened up for public use.

tvOS

Bring back some API: We get to use a lot of the same frameworks on tvOS as we do on iOS, but there a lot of frameworks that seem like they could work great on TV, but aren’t available. First on my list: MapKit. There are a lot of cool things we could do with maps and points of interest. Because WebKit isn’t on there, Google Maps is not a viable alternative. I have resorted to Google’s static map API to at least get an image painted on the screen but that is not rich nor interactive.

watchOS

Custom Watch Faces: Even though I love my Apple Watch, apps terribly suck and they likely will for the foreseeable future, mostly because of anemic hardware. One thing that I would absolutely adore, however, would be custom watch faces. I doubt this will happen, however.

UIKit on watchOS: This is more fantasy than wishful thinking, mostly because of the underpowered nature of the hardware. I'd really love to be able to create more interactive controls and beautiful animation on Apple Watch.

macOS

UIKit on Mac: Rumors abound about UIKit being brought to the Mac. This would be pretty incredible, actually. I have resisted making any Mac applications because of the antiquated AppKit and some of the convoluted things it brings with it. Having a familiar app framework would be pretty amazing and porting iOS applications would be a huge thing for being able to kickstart what is an App Store devoid of really great apps. I realize that making UIKit apps for Mac might actually entail completely different UI code, but at least the frameworks and patterns would be the same.