I want all the little things that are available on the iPhone but missing on the iPad/Mac to make their way to those platforms.
Specifically, the Weather app, Health and Fitness apps, Calculator app on iPad (!!), Wallet app.
I'd also like a real Phone app on the iPad and Mac for continuity calling, with an actual dialer, synced missed call/recent call longs, and access to voicemail via iCloud syncing or something like that. I work from home now and sit in front of my iMac and iPad all day and accept calls on those devices all the time, but not being able to listen to voicemails and not having a normal dialer is a huge pain. And having all that stuff awkwardly mixed into the FaceTime app isn't very intuitive, IMO.
I'd also love it if the Clock apps were in sync between devices (and had a Mac version). I bounce back and forth between my iMac, iPad, and iPhone all day long and way too often I start a timer or alarm on one device, only to move over to another device in the interim, and either miss the alarm or have to run to another room to stop it. It'd be nice if the alerts come through on all devices and could be dismissed from all devices too.
I'd like live streaming services to be integrated directly to the TV app. For example, I'd love it of all the live channels I get via Fubo were viewable/accessible directly via the TV app.
Finally I'd love it if we could see the battery levels of all iCloud connected devices in the battery widget of each device. So that I can check my iPhone's battery from my iPad, etc.
Edit: I forgot I'd also love it if all iCloud-connected devices show in the Now Playing tab in Control Center. So that if I'm playing something in my iPhone, I can see it/control it as a tile in the Now Playing widget on my iPad, iMac, etc just like it were an Apple TV or HomePod.