I'd love to see UX improvements around minor inconveniences of the iOS experience:
- what the hell the volume button does in different contexts (sometimes it changes the ringer, other times the in-app volume, other times it just seems to go into a black hole)
- easier access to app settings for the app you're using (things like privacy settings and notification settings and revoking/re-allowing privileges (like camera access, etc.); maybe context-sensitive pull-up or pull-down things? I like that OS controls this, not the app, just need a better way to navigate to it.
- turn off buzzing and booping when I plug in a charger, and maybe some sort of wireless charging thing if the technology is where it needs to be yet
- stop showing album art on the lock screen (or make it optional)
- more uniformity on how lock screen notifications are acknowledged and dismissed -- sometimes there's an 'x' in the swipe-over, sometimes actions, sometimes just pressing the notification does something
The only feature that seems interesting to me is the voicemail transcription; previous services I've used for this have been pretty terrible, but hopefully this is better.
With Marshmallow's security model (of run-time grantable and revokable permissions) Android has become a lot more attractive to me -- my biggest pain point in using Android devices has always been the nightmare of looking at what an app wants in order to install.