Add a feature which lets us tag an individual track in the music library and a movie with a “Theme Song Tag” and the movie will use the track as its theme song when on its details page. TV Shows already have theme songs by default, this feature would make it very easy to add them to movies as well.
You can already add local themes to movies same as you can for tv-shows by adding a theme.mp3 file in the movie’s root folder.
There’s an existing feature suggestion asking for an option to manage various local extras (incl. themes) using the Plex Web interface
There’s another feature suggestion asking for an option to include movie themes from an online source (considering to use songs from a movie’s soundtrack etc. via Tidal)
Do I understand your suggestion correct that you primarily want an option to assign a song from a music library of yours to a movie?!
2022 clean-up: considered implemented (movie libraries support local themes) / duplicate (see above)