I've started to make use of MQTT to monitor devices (though not yet for control). I hesitated because of the potential of having a single point of failure, but as a monitor, that is no problem and far more flexible than the current use of HTTP for notification. It would be helpful if Hubitat could act as an MQTT client for notifications and, optionally, as an MQTT broker.
Separately but closely related is the use of JSON for settings. The maker API parameters on the command line would also be encoded as JSON to allow a message to be sent via other channels (such as MQTT), thus leveraging the existing drivers.