Let's just think about this a bit more:
8 gates, but up to 64 with DOUT/DIO_MATRIX.
DIN SYNC is 1 start/stop but up to 7 clock divisions/multiples.
AOUT is really 6 signals (Vs, Vd, SO(SI), SC, RC1 and RC2) but Vs is common to the Core and doesn't need a separate ground.
1 sync input to control the master clock/ forwarded to the additional clock outs.
What about analogue inputs? These would be pretty cool to interface from a modular system back to the Core. Diode clamps to protect the input pins?
We're getting into quite a few signal lines now, too many for a DB-15!
But for a basic usecase:
8 gates
5 AOUT
1 clock start
7 clock out
1 sync in
8 AIN
1 signal ground
31 pins ---> DB-36/Centronics?
With only one clock out you'd be back to DB-25.