Na das klingt doch mal interessant...
Stereo or Dual sid possible.
Dual pin pwm, 5bit+5bit for 10bit with less r/c filtering.
X/Y paddle adc.
Midi in/out, uses $d41d and $d41e
wildcard at $d41f, like small program download/firmware upload.
and for permanent settings.
Pad 5 and C(with analog) can be used for diy stuff,
like a wire for dual sid or a wire to Restore key that if you hold it down for 2sec it will pull C64 Reset low etc.
I will send a way for 3 pcb proto today, should have something built in 2 weeks,
then later I will start selling them at $29, software probably will be a group project.