Well, I think it could be transformational in two ways. They could finally show some games that would have big appeal to all gamers thus transforming the image of the xbox line being just for mature or just for hardcore or just for shooter fans that have plagued xbox consoles since 2001.
Or they could announce the 720 and the reason that it has been so quite lately on the 360 game development front would be because key developers have been working behind the scenes on 720 games and they will finally be unveiled at E3.
But, I wasn't anticipating the next xbox for a bit longer, but if you do look at the changing of the guard from original xbox to 360, then you will see that the 360 is in a similar spot in its lifetime now to when the 360 was announced in 2005 (4 years into the consoles lifetime, Halo 2 released November 2004, Gears 2 released November 2008, etc.)