Please add libmpv backend to phonon: https://github.com/mpv-player/mpv/tree/master/DOCS/client_api_examples https://github.com/mpv-player/mpv/tree/master/libmpv http://mpv.io/ Reproducible: Always
If you find someone willing to write the backend code and maintain it we'll gladly add it to the backend pool and providing release management and whatnot. We (that is the present Phonon team) are not going to develop nor maintain any additional backends though as that would take time away from other tasks.
Recently a developer published a libmpv phonon back-end based on libvlc, is not feature complete at the moment. https://github.com/OpenProgger/phonon-mpv Worth a look.