Feature: don't overlap music players
A feature that is becoming widely used in mobile OSs such as Android and iOS is to pause currently playing music clients when a new one starts playing.
This could be implemented in g-s-d as well for all MPRIS supported players in a relatively simple way: when a new MPRIS client starts playing, or when an already open MPRIS client switches status from "pause" to "playing", all other MPRIS clients are sent a signal to pause the currently playing content. This feature would allow for a much more modern music playing experience on both desktop and mobile devices.
A dconf flag for this might prevent possible side effects in edge cases.