This is really helpful for DJs on the Spotify desktop app.
Instead of taking up screen space for all users, you could make it an option in preferences to toggle the display of tempo (bpm) and the key (root note) of all songs. Additionally, you could provide the Camelot notation for even easier use by DJs or anyone interested. The features provided by Djay 2 are nice but constraining.
It wouldn't be too hard implement (key and tempo can already be retrieved with an Echo Nest API call) and a great feature.