There are still people who like the idea of a dedicated mp3 playing device (vs always using their phone for that), but I'm not sure those same people are the on

There are still people who like the idea of a dedicated mp3 playing device (vs always using their phone for that), but I'm not sure those same people are the ones that care about having an open source version of that device. Old mp3 players are readily available and cheap. That said, start a project and document it well and I'm sure you'll find others that are interested.