Create a sandbox where web-based apps can be installed. Specifically, apps like fully functioning Hangouts.
No cost or very little cost of development.
Moderator: Actually this would be a huge software development effort. The Tesla Model S is not based on Android or Chrome OS. In addition, due to very restrictive regulations in vehicles (at least in the USA), it would have be designed to prevent apps from showing animation or video or even flashing colors. This is a non-trivial effort and any hack that bypassed such limits could make Tesla directly liable. Even offering an SDK and allowing custom written apps is a huge project in itself for Tesla.
Tesla has stated informally that an SDK and third-party apps would become available at the end of 2015, but that seems doubtful. Nothing official has been stated since that announcement by Elon in 2013.