Detect key signature or musical key of song
I think it would be great to be able to detect the musical key of the song and offer the user a way to write that as an mp3 tag to the file.
This is something that is immensely useful to djs who do harmonic mixing, where changing the key of what you're playing up or down certain steps will change the mood of your mix.
I like it. I believe AcousticBrainz might do this, e.g. https://acousticbrainz.org/7eef7a92-e282-4cf7-89a4-c35d39ff3d4d .
Also see (for different types of mixing): http://ideas.blisshq.com/forums/21939-bliss/suggestions/1475605-update-bpm-field