If you want to set up the frequency spectrum of your instrument, but not fixed inside the spectrum, a keytrack mode for a selection of bands would be nice that the EQ-ed instrument behaves relative to the played note. Beside that you want to rise or lower the overtones of an instrument, an overtone detection would be nice. These two features are really helpful in a musical way.
As example: You can raise up the overtones, but lower everything else (it could be controlable with one knob which rises the overtones and lowers everything else with the + direction and with the - direction it lowers the overtones and rises everything else(bipolar knob). After that turn on the keytracking mode, and it follows your played notes.
If you can load frequency masks like emphasised harmonics 5th, 7th, 3th, octaves...... or you can save detected overtones from your audio material, this could be really powerful. You can make rich audio material musical like a noise and you have something like a synth if you want to call it that way.