As a backup option, the incumbents already make this. Carrier Infinity is a good example. Proprietary protocol (RS-485 based, so it can run over existing thermostat wiring) between the smart thermostat and the furnace, and between the furnace and outside condenser unit. Even with all these smarts, there is still a fallback mode provided with a regular old dry-contacts control wires that can hook to any standard thermostat. Yeah, the system won't operate at peak performance, but it will operate, and that's very important when every HVAC tech carries a standard thermostat with them, or even uses basic jumper wires, in an emergency until they can return with correct parts.