Driving stepper motors is a perfect example: you can grab a cheap controller board, hook it up to a microcontroller, and the rest is essentially just software.

Klipper can control multiple microprocessors with no trouble and keeps them in synchronization, so you could have a processor for your extruder and one for each stepper, for example.

AutomationDirect is a distributor offering thousands of industrial automation products for electrical control systems, including PLCs, operator interfaces, AC drives, motors, stepper systems, sensors, ...

Why this hasn't been done before is a mystery to me. This little hack project by Kellbot of NYCResistor takes a rolling ball, an optical mouse, a gutted PS2 controller, and some Arduino hacking ...