Another vote for open thermostat.
Can you tell us any more about the HVAC power board you have in the works?
I hope you’ll over provision it a bit as I want to modify it to control more than just a normal furnace/AC.
I need a few extra outputs to drive 24v relays to control dampers and my attic fan.
I also need some extra temperature sensors (preferably both RF and a network of ds18b20 one-wire sensors).
It would be great if it also had an expansion board connector and expansion boards for both 24v controls and relays.