my suggestions :
make the base unit pretty much like you have it.
have a side connector for a second unit, IC2 or some such and allow a second unit to have dedicated parts.
if you look at any of the small PLC's on the market, they have one base unit, and then add-on units.
that keeps the main unit clean and gives everyone the option of stuffing the add-on with all the circuits they want.
I am looking for dimensional layout of the terminal board. that way, I can make my own for my needs.
Dave