expanded view of the harmonic editor,
a log/lin mode for editing. It's exceptionally fiddly when a partial is 2px high and you want it to be 1px high - the current editor needs more precison at the lower db end
some kind of mathematical operator for modulating the harmonic series (automating 64 partials with an envelope for each one is too crazy, but a mathematical operator on the series would be fun.)
Imagine a random percentage on the harmonic series for an oscillator, 5% variation morphing every 100ms. nice harmonic movement.
a sine animating the series would make anything from phaser/flanger effects to physical model type stuff.
