The thing I'd like to see with iOS JRemote more than any other thing is proper support for home-buttonless iPhone screen sizes. On my iPhone 11 Pro and my wife's iPhone X, it is still letterboxed (all these years later). This is, I'm sure, related to the mentions above of pillarboxing on the home-buttonless iPad Pros above. And since, with the release of this year's devices, those sizes have now moved across the device lineup, it has now moved to the "mainstream".
That seems like a reasonably minor adjustment, and it would really be nice to see it.
Otherwise, of course, I still really, really want the ability to use JRemote offline with locally cached files (where you can pick a particular Playlist and it automatically syncs those files to the device nightly via Background Updates, and then has a special locally-cached copy of the Library it uses when it can't get online). I live in a place with very spotty cell reception, and JRemote is all-but-useless for music playback while I'm driving around. But that is, of course, a bigger ask.
For now, I'd take it just using my full screen.