An interesting feature (albeit a niche one) would be a volume peak search.
It is a feature that cd players had in the '80s and '90s in order to tune the volume of the cassette recording.
This is how it worked: pressing a button the cd (or selection of tracks) was silently read at high speeds and when the highest volume peak was found the player looped the six seconds around the peak until the user stopped it, so that one could adjust the maximum rec volume for the cassette deck without clipping.
Another useful feature, in addition to that, would be to normalize all the tracks in that playlist to that peak in order to set the recording level only once.
Nowdays the tape is coming back and many people love to experience again that vintage equipment. It would be cool to have a mix between hypermodern open source tech and old analog excellence.