Do people want this? Maybe I want both warmth and fresh air in the room? And I foresee the day where the window sensor is faulty. I'm shivering in bed as I have an argument with my furnace about which one of us should be in charge of setting the temperature in my bedroom.
Hass just sends on/off commands to a self-contained thermostat in response to window sensor state change events. If it fails, you can still manually go to the thermostat and set it to whatever you want.
https://www.home-assistant.io/integrations/venstar/
I've not used it (I installed one because I wanted scheduling and liked that it had further possibilities and wasn't overly expensive).
For a more advanced use case I’ve recently set it up so that the thermostat logic is within HASS. An automation driven by a binary switch helper then switches the thermostat to a fixed hot or cold value for heat and AC. Then I can feed the HASS thermostat with whatever temperature sensor data I want, like an average of multiple sensors around the house. All this is totally optional though if you just want to control the thermostat from within HASS.