Admittedly, this idea needs a bit more than Google Maps, but it definitely needs your car's windshield.
Any web service today which offers any kind of geo-spatial data becomes useless if you can't get "there" easily. Sure, my cellphone can probably show me a tiny map with directions to my destination, but a hands-free method would be much better. And much safer when I'm driving.
Scenario: A wireless device in your car is sent a message from your favourite geo-app. This device illuminates the interior surface of your windshield with a street overlay. From your perspective it looks like your route has been painted onto the street with your favourite colour. Choose a green line, a red dashed line, a thin yellow-bricked line, and follow it, left, right, and straight to your destination.
Safety issues? Sure, there are probably many. But I'd rather be looking straight ahead at a cool transparent green line, then constantly referring to a small unreadable map terminal to my right (or left, if you're in Japan, England, and all those other cool countries.) These issues can surely be dealt with.
This could be extended to (and I think some engineering groups have already done work in this direction; again, I wish I had become an engineer!) glasses, sunglasses, contacts, motorcycle helmets, planes, trains, etc.
Where's the Web 2.0 element? Coming in the next post. Stay tuned.