Yes, they all look really interesting, also a subset of this is waveset synthesis that I like the look of also.
I think you can split the current synth engines on Norns down by synthesis types and look at the obvious ones missing. Wavetable synthesis is one - like a Piston Honda / Buchla 259e / PPG Type with a combined low pass gate (like the Passersby back end).
Maybe an additive synthesis engine like the Harmonic Oscillator also. Theres other less well known ones like Waveset synthesis (a little bit like wavetable) like this https://www.yofiel.com/software/cycling-74-patches/5-5k-waveset-oscillator and this https://vimeo.com/158675851
Also Pulsar synthesis intrigues me (from Alberto de Campo and Curtis Roads) like this https://www.curtisroads.net/software/ and this https://www.marcinpietruszewski.com/the-new-pulsar-generator .
Even a Phase Distortion Oscillator which I personally much prefer the sound of compared to FM.
I also think that you only get the full benefits of these sound engines when there is some dynamic modulation involved. Currently all the parameters can only be addressed by cc. I would like to see some sort of onboard modulation bus containing lfo’s, envelopes and random to control the parameters section of engines. Molly Poly and Passersby have a mod bus and it makes a big difference.
I personally want something abstract, random, chaotic, atonal, not necessarily melodic but with dynamic timbral changes… rhythmically interesting and challenging. Sounds that you listen too but can’t quite work out how they were made, but are intriguing. I like stuff generated from random / shift registers - things like the Blippoo box / Benjolin, pinged filters, resonators, phase distortion, wavetables, that sort of thing, not necessily noise for noise sake, just something genuinely different that is currently not on Norns.
The current sequencers types on Norns/Teletype/Ansible are really interesting and different. You will not believe how many people have asked me on social media about Loom for example. Nobody has seen anything like it before. I want a sound engine that has that kind of impact.