This may be an edge case, but it would be nice to have a accessible in the Multimedia Control UI a "Playback Device" drop down to switch playback device on the fly for each application. This functionality is available in the latest Audio Volume Settings on KDE by dragging an application to the playback device. The use case for this is, oftentimes I'm in my room, away from my desktop computer and I want to switch audio from my headphones, to my bluetooth soundsystem in my living room.
With https://invent.kde.org/network/kdeconnect-android/-/commit/a4b3da7a14d0c56cdabbc0f0af6f8ac037e89d0b it's possible to switch the audio output (although not on a per-application basis)