I too would be interested in purchasing OpenThermostat especially if you create a version for the Pi. For what it’s worth, here’s the list of features I would find valuable. I’d be interested to hear the features others would like to see.
Powered by the HVAC system — the same 24 VAC that powers other digital thermostats and OpenSprinkler.
Onboard real-time clock (like OpenSprinkler) in case the internet connection dies
Switches 24 VAC to control heating stage 1, heating stage 2, A/C, Fan (blower) from automatic to always on
Connects to one or more wired or wireless temperature sensors
I know this sounds extravagant, but self learning algorithms are really important for high-efficiency HVAC systems. Without self learning algorithms it’s difficult to know when to schedule the heat or AC to start in the morning because it can take an hour to raise the temperature by 1 or 2 degrees.
Mobile app to display current temperature, override current scheduled temperature and control scheduling.
I would suggest that rather than attempt to develop a slick looking box to mounting on the wall (think Nest), it would be better to located the OpenThermostat in the furnace room or unfinished space. Slick looking boxes are expensive and people (like my wife) are picky about how something looks on their wall. By mounting OpenThermostat in an out of the way location, the appearance of the box won’t be as important and the mobile app can handle all the functions.