The projector joke (?) got me thinking of this
Discussion
Projector would actually be a really cool feature for a car but I believe more phones should be like this 
I'd also carry a folding keyboard/trackpad thing separate from the phone so it can be turned into a computer. Those already exist
Sorry I missed your call, left my projector at home.
Reminds me of this usb second monitor thing I had a while back. Was decent to add to a laptop, but a bit dim and ultimately I stopped using it.