I have an initial prototype board, which is basically an OS3.0 main controller board plugged into a OpenThermostat driver board (it can be powered either by USB or 24VAC if you have a C wire from your HVAC system — this is pretty much the same as OpenSprinkler’s 24VAC power circuit). On top of that it currently has two relays and a temperature/humidity sensor (which is on the OS3.0 main controller board as there is a 4-pin header for AMS2320 T/H sensor). The software is very primitive at the moment and needs a lot of work.
You said it would be great to also have expander connector, which I haven’t thought about — in a typical HVAC system you only have a few devices (like mine only has heating and cooling, not even a fan). What would expanders to?