Adding some sort of absolute linear encoder (like BartK’s linear slides, above) would be cool, but integrating the inputs from those and motors is way above GRBL’s abilities. I think some people have done similar with LinuxCNC, but a minimal LinuxCNC controller with modern hardware is around $500. In theory that could drop to ~$200ish with a RPi 4 as a controller, but I don’t think it’s really stable enough yet.
I have no idea how much those HiWin encoders cost, but I’d be amazed if a full set of them could be had for under $1k. I’d guess that they’d be closer to $1k each. I’m not really sure what that’d buy you in an aluminum-extrusion CNC, given how flexible the frame is. There’s no point in being able to measure axes to the nearest micron when the whole frame can flex 50-100 microns.
They’d be a great solution for the “my homing switches aren’t accurate enough,” though.
Monitoring spindle + drive power is probably interesting, though. I’m in the middle of moving my HDZero from GRBL to LinuxCNC, and I’ve included spindle and individual stepper power monitoring in the build. I should be able to collect power stats at ~1 kHz (although I’m not sure how useful that is with AC). That should be enough data to detect things like burying a 1/4" endmill in aluminum before the motor completely stalls and breaks the endmill, but I’m not sure what I can really do with that data without creating a false-alarm problem that’s bigger than my broken-endmill problem. It’ll certainly be useful for monitoring and optimizing cutting performance, though.