Here's an Idea...

An LED lit marquee that you would mount in your back window.

It's light enough and thin enough that you can use suction cups type window mounts that would hold a scrolling marquee in your back window.

It would be wireless or Bluetooth and would connect to your mobile device with an app that allows you to call it up with your voice by name that would issue commands you have programmed.

Like, Have a Nice Day :-) or Get Off my ASS! to Game Day! that has a rolling message about your team with stars and balloons or fireworks like digital image. ex. TOUCHDOWN!!! ;-)

In reference to Get Off my ASS! we see a Kinect like eye that can judge the distance of the car behind you to let them know if they are too close to your car.

It could have the feet left to stop or it could even count the car lengths at highway speed that would let people know when they are at a safe traveling distance.

If you travel within the safe limits the readout would thank you and then disappear. It would only reappear if you come to close or you activate it.

The Sporting Banner is what we see cheering on your Team GO TEAM GO TEAM like marquee running across that other people can see.

You could even like the banner if the other car had the same app installed that would send a token of appreciation to the driver. It could also go directly to your online profile like Facebook to share with others.

We see it being fun, flirtatious, angry really anything, maybe even prosperous...it could be a fact!

We actually see it becoming integrated into the back window itself. It's not any different than what they have now with the Dash Board Gauges in the Windshield...