Thanks looks interesting.
Some thoughts and comments:
1. It would be nice to see an electrical specification i.e. what are the ON/OFF voltage levels for the inputs, what are the drive capabilities of the relay outputs etc ... How would these compare to a PLC
2. I am not too familiar with analog outputs on a PLC is a PWM analog out clean enough?
3. I would suggest some clamping protection on the inputs, especially the analog input and the digital inputs. I would look at using transrobs maybe the SMBxJ range?
4. What form factor will the board be in?
5. What kind of connectors are you proposing to use?
Just some early thoughts, but looking good.