Damir
Thanks, Damir.
I'm glad I'm not the only person who can see some use in this. You are right that trying to do this mid-performance with current filtering options is almost possible but difficult and that it needs the user to structure and categorize songs meaningfully to work but if we are willing to put that effort in, I think it could be rewarding.
A simple implementation is maybe a single, big button that cycles between genre (a new field like 'keywords'), bpm, and off and every time a song loads (or the button is pressed) the filter is updated based on the new song's genre(s) or tempo (+/- X%). Cycling the button to 'Off' just disables the filter and shows all songs with the default sort option. Seems a lot to gain and you're no more than two taps from where you were without it.