Personally I think its unlikely that there will be a retractable hardtop for the normal Bluestar. These systems are expensive in terms of cost in develop and production, and in terms of weight, and Tesla will certainly be wanting to keep both cost and weight low to meet the 30,000 price range while giving as much driving range as possible. The Bluestar is also supposed to be Tesla's "Car of the People" and convertible hard tops usually take up a lot of trunk space (or all of it), and also pretty much require the vehicle to be a 2 seater, not a family car.
I could definitely see this for a future roadster based on the Bluestar platform though.