For now I plan to make two basic one:

• isolated input module
• relay output module

when it is star to work I want to add some more advanced one:

• digital output
• analog input/output
• counting/generating signal modules
• sensor module (some kind of ADC with enough powered to calculating sensing result - for example FFT with vibration sensor)
• communication module (LORA, RS232, RS485, Modbus, CAN and so on)
• audio module (some module to generate simple music tones)
• motion module (module which interpreted G-Code and drive axis) 

do you have any idea what more could be done?

For sure I want to make all device with multi-controllers setup. So one with micropython will talk to each other, and others only make one task.

Main challenge right now is to design good concept of internal bus to realize all of this functions. For now I think about some SPI with some additional interrupt and synchronization lines.  I will be very thankful for any suggestions.