I would say a host of physical contacts offering no/nc options as well as the potential for RF connected sensors as well.
I figure since you are already working on a thermostat idea the two could be paired together so there is a little more intelligence for the HVAC system.
Having little kids it is rather annoying to run the AC/heat and discover someone left their window open all day long, this is only going to be more of a problem as they get older.
Maybe an overall schema where all of your platforms can communicate together, OpenHome, OpenHouse, or something similar?