I don't even know how this would work, but Xbox Evolved says that the next Xbox (720 or Next, whatever) won't just be backward-compatible with games, it'll be forward-compatible. What the hell does that mean? Forward-compatibility would mean that games made for the 360 will be able to take full advantage of the next Xbox's muscle to look more amazing and like that generation—not just cleaner textures and higher resolutions, but better frame rate, draw distance, the whole deal. Also, it would mean features in the 720's controller would be used in the game as well—in effect, yanking it forward a generation.
Xbox Evolved says that Microsoft is "testing the waters" on the next Xbox's final specs, but wants to put this option out to developers sooner, rather than later. The whole thing sounds a bit too good to be true (meaning it probably is), but they say that it's from one of the same sources that told them the Xbox 360 hard drive would be optional long before it was official, as crazy as it sounded at the time. [Xbox Evolved via Xbox Scene]