Unless they somehow really mess up this game (unlikely, every game Sakurai has touched that I have played feels like gold), I feel like Nintendo already won E3 for me.
I stick by my prediction of them showing off HD graphics on old characters (Link, Mario, Kirby and Pikachu, most likely … maybe Pit considering it's Sakurai), along with new character reveals and new features. I'll add to it that I imagine they'll explain the difference between 3DS and WiiU versions and how they can connect, and end it all by revealing another huge third party character. I would hope Megaman but I can see it being someone from Namco Bandai.
It also seems like Sakurai is looking into which Gen VI pokemon content he can include in this game. I must admit I fangasmed upon seeing the idea of Sakurai and the team of Pokemon X and Y sharing information. I don't know, I guess after a couple months of doubting Nintendo with stuff like NSMB2 and Paper Mario Sticker Star, within two weeks they go and show me why it is they own my childhood and my soul as a gamer. I seriously feel like my younger self right after an E3 presentation.