Coming back to local Linux music playback, I'm pretty disappointed with the state of things. Spotify's UI feels nice in comparison, which says something, given how neglected it seems. Things largely feel like 2016 or so. Every music player seems to solve the same problem using a totally new codebase, with only low-level libraries shared between them (reading MP3 or FLAC files, for example).

I can't be the first person to have this idea... but, do like neovim did for text editing, like the Language Server Protocol does for coding: split apart core music playback functionality in a UI-agnostic fashion, and let interfaces interact with it via a socket protocol or whatever the equivalent of that is nowadays. (D-bus? MessagePack? no idea)

Handle the file formats, indexing, tag editing, output interfaces, notification of events, maybe interaction with online music repositories (e.g. FunkWhale)