I'm actually looking to add simpler waveforms in some ways, at least less complicated than the included saw and square! A bandlimited triangle would be a nice one, I really like the sound of triangle-on-triangle FM. Just a bit more complex than sines alone.
I also want to add (softly) bandlimited saw and squares. Hard bandlimiting (where the harmonics are normal level up until the point they suddenly stop) sounds kind of like it's has a resonant filter on it (I guess your ear hears the top harmonic as a resonant peak). I'm plan to do some soft versions where the last few harmonics drop off a bit. The other ideas I had were simple waveforms with the fundamental and just one or two harmonics.
I have written a few additive waveform generators in Jesusonic so I just need to work out how to export them in the right format and set up the build environment for the firmware.
I remember you originally saying that the waveforms had to be in the quick RAM and so adding some required removal of others, but it's nice to hear that it's not the case any more and we can just append them. I feel bad suggesting this because you already implemented one of my suggestions really well and really quickly, but what about an option to load a custom waveform into RAM from a folder in the USB drive? It would mean new waveforms could be added without building custom firmware and reflashing. Of course, I might be the only person that would use it so it's possibly not worth it.