You have the ability to specify a tachometer pin and it displays the RPM of the fan. But it would be fantastic for it to enter an error state if the fan speed enters below a threshold % value.
It’s more complex than that for sure, because each fan’s RPM curve will be different. But in the case of a hotend fan, I want that to always be running at a fixed speed when printing. And nothing worse than failing a print due to a fan dying mid print, and having to unclog that seized up hotend 
Perhaps you can specify tachometer_failure_rpm and it can be relatively low, like for example 1000 if your fan is capable of 5k/rpm.
Or perhaps even calibrating the fan and taking a range of normal RPM speeds at % speed values, and if it deviates from that for X seconds, it’s a failure state.
How do you add motivation to this feature? It’s something that could have saved me lots of pain.