If we go for an alternative, the main requirement is surely #gtk4 / #libadwaita with adaptive UI. For distros it's probably also important to have some kind of dependency management to install missing codecs.
The language could probably be anything from c, rust, vala or gjs.
I wonder about the backend - should sticking with #GStreamer be a requirement or is something based on #ffmpeg / #mpv ok as well?