I know it’s not NETMF, but I went with a Radio Thermostat ($99 Home Depot version), and I love it. It’s for “normal” residential systems, communicates over WiFi, and the big plus, while it can be managed over the internet, it can also be managed locally, and requires/uses no subscription services.
A residential thermostat though would be a fun project. It’s not much more than a temperature sensor, a 24VAC power supply, and a few small-signal relays.
Here in the US, “normal” means either a heat pump or gas-fired heater, and possibly an electric AC unit. The thermostat is a simple controller that makes or breaks 24VAC connections in certain ways to call for heat, cold, and/or the fan.
The comunication protocol is called “OpenTherm”… so basically the two wire connection from teh thermostat to the heater is not just an “on/off” control to open and close the heater but it is more complex…