I'm planning to make it simple, like just wiring up a PCF8591 on the Pi's I2C bus, which has 4-channel analog in and 1-channel analog out. Obviously this chip works with low voltages, so I might add some circuitry to amplify the range to 0-10V which is very common in industry. Do you have any suggestions in mind?
About the case, I don't plan to make it fit on the NEO's case. I might just ask Petri to design a square case for it. I'm designing it to be just a single board with the connectors on the edges, so a square casing with DIN rail mounting on the back should be a good start. However, when I make it available for selling, the case will be optional, so that users that just want the PCB can buy it with the lowest cost.