Having a variety of waveforms would really broaden the tonal and textural palettes of Tremolo, Phaser, Flanger, Chorus, Vibrato and anything else that uses a Low Frequency Oscillator.
It could be implemented with a series of vertical sliders in a similar way to the Korg NuVibe pedal.
https://www.youtube.com/watch?v=1IxzhWucRcc
This way the user could sculpt their own wave shape.
Anything from a simple sine-wave to something very complex.