I don't want to call your brain stupid, but an HD DVD2 would be a quick failure, especially if the disc could only hold 250GB, when Blu-ray discs have already be made that hold 400GB, on one side to boot.
The only problem with the PS3's Blu-ray drive is it's only 2X and by the time the next generation rolls around 12X-16X drives should be available, if not faster.
If there is another physical format for games (which I'm pretty sure there will be) it will be Blu-ray.