A module that snaps the partials to fundamental * a. The way i would do it is:
mod = (fundamental * a * somebignumber) % (fundamental * somebignumber) / somebignumber;
freq = freq - mod;
Except for the first partial for obvious reasons. a is a slider the user can control. Hope that makes sense.
This has 3 great use cases i found.
1: Changing the partial frequencies can be used as more of a harmonic timbre control if you quantize them with a = 1.
2: Using custom scales and stretching the harmonic series accordingly can bring things back in harmony since the harmonics overlap.
3: Better controllable dissonance.