I’ve looked at both Koel and Mopidy both of which kind of do the job, but don’t have native clients and are honestly no better than just using Plex for your MP3 collection(s).
Given all the advantages that a Plexpass has (with audio streaming) It’s kind of a no-brainer releasing a native audio client for mobile that does not require using the Plex app (think of it like how Facebook split out Messenger into a seperate app a while ago).
Search first if the request has already been posted before. The goal is to collect the votes for a particular feature in one thread - not to scatter them around.