Sorry to be out for so long. After reading some of the comments, I just wanted to add this in. Look at what PD and Gran Turismo have done without Ferrari, Porsche, Lamborghini, Bugatti, McLaren (at least in full detail), or any other well-known, established sports car companies. The issue of not enough sports cars in the game was a concern, but no reason to completely turn away from GT. They proved that while these companies are nice to have, they aren't needed as long as you can still deliver a solid racing title appreciative of many audiences. As redundant as all of this is, it's true. Not even Pole Position acheived this much attention and fame that the GT series has. Pole Position was more of a front-runner for the future of racing games. Then came Ridge Racer. Then came Daytona USA. And a few years later... then came Gran Turismo. Love or hate Gran Turismo. At least it's done more than any other racing game in history, not only in video games, but even real life.
PD will need to think about something special to have a celebration. When you celebrate 10 years of something special, you need a quality celebration. You just HAVE to, especially what this game series means to the video game industry. I'd probably go on record of saying that the racing game genre would be dead and lifeless had it not been for Gran Turismo. You never know what you got until it's gone.