These are what I can think of that I'd like to see, in no particular order:
- Battery widget that displays battery status for all Find My devices instead of just those connected over Bluetooth
- Apple Music playback state sync between devices
- NFS network share support on iOS (SMB is a major pain in the ass)
- Advanced audio routing: output to multiple devices at once on iOS/macOS; play from multiple devices at once on AirPods or chaining devices, e.g. iPhone -> Mac -> AirPods (I already do this with my Linux PC instead of the Mac)
- Sleep mode on Apple Watch to show normal watch face while still having the screen locked
- Subtask support for CalDAV reminders