For me personally it would be useful to be able to lookup chords by their middle and upper notes. Let say I improvise a lick and want to find some harmonization. F# and A should evoke not only F#m but also Dmaj and Bm7, and maybe something more spicy like D#dim7.

Another useful feature I would appreciate is to see possible functions of chords. Dmaj can be a I of D major as well as a II of C lydian. So as I lock the key, let say C Lydian, the tool might highlight other notes which are in key. That’s helpful for improvising further or when looking for a chords progression.

I’m pretty sure there are DAW plugins doing that but a simple web tool can be used in so many situations when DAW is not there.