Tips from the dev Edit
Layer 1 is always processed even when its turned off. It still can be used as a modulation source in layer 2.
There are 2 interesting modulation destinations: mixer input and amp input, so you can still mix noise, layer1, envelopes, lfos into audio signal path, or bypass the filter. Like OSC -> FILTER + - OSC = you can get HPF, shelving filters and everything in between.
I like self AM: osc 1 -> osc 1 mix on saw waves - you get a whole bunch of imperfect saw waveforms from different vintage synths. As about sawtooth: set waveform to something between saw and pulse. Set pulse width to 100% (inaudible) and use constant PW modulation target = CS-80 saw.
And remember: slots can modulate each other.
If you have an AU host app that supports MIDI learn such as AUM, you can already use MIDI learn for Zeeon.