Some thoughts for directions on control logic and oscillators.
Focus will be on the Arduino microcontroller in this post.
I like the Arduino Pro Mini. They're tiny (similar to a pinky finger in size), cheap and I have several of them, which helps.
Firstly, perhaps most importantly, should it have any control logic ?
In my view, yes, for MIDI, routing, digital waveforms and perhaps global FX.
Should it have digital waveforms ?
Tricky. I would much prefer sound creation to be purely analog...but do have experience of bad wandering and the need to warm up the circuits until they behave.
It would seem a good idea to have digital oscillators. Though, nothing wrong with additional oscillations coming in from such things as Lidmotors Penny !
Should it have a screen ?
Too much of a bling or useful ?
That one is a maybe. It could take away from a pure look, but, add a lot of quick to check settings, operating values etc.
I have a 2.2" 320x240 65K colour screen that could be used. Or a tiny 0.96" OLED - too small in my opinion for ease of use.
Have screens been useful to you, on your gear ?
Present Arduino based Open Source systems that could be used, in full or part. For their functionality and to not reinvent the wheel in several areas.
MeeBlip anode:
http://meeblip.com/Code:
https://github.com/MeeBlip/meeblip-anodeGood monosynth already !
Mozzi:
http://sensorium.github.io/Mozzi/Nice clean sounds and very flexible library.
AVRsynth:
https://www.youtube.com/watch?v=v_j8ymWVYu8A monosynth with a good external controls line up, MIDI In
Octo-Synth:
http://www.cs.nott.ac.uk/~jqm/?p=6058 note polyphony, 16 oscillators !
Granular Synth:
http://www.notesandvolts.com/2014/07/build-auduino-granular-synth-part-1.html2 oscillator rumbler, simple, pot controlled (until keys replace the pitch pot)
'Nastysynth':
http://www.msarnoff.org/1bitsynth/Video:
https://www.youtube.com/watch?v=fDE_n_OJ8ScThe demo of this thing is breathtaking. Video quality is rubbish, but the sounds are perfect renditions of every coin-op arcade machine of the 80's lol
More options:
http://playground.arduino.cc/Main/ArduinoSynth