This E3 will be HUGE. New consoles from the rivals, but I'm still most looking forward to the Nintendo stuff.
I feel like we've just had a mini E3 this week with all those announcements of new games anyway. I'm still hoping they have a couple more Wii U games under wraps (please be working on a starfox game!!!), and we know literally nothing about 3DS other than Pokemon! Pretty much every game we've got coming in the next few months from Nintendo is already out in Japan (Fire Emblem, Animal Crossing etc) and we know NOTHING else about whats coming. I expect Mario will have another sports game announced for 3DS, but apart from that it could be anything. Something different from the stuff being worked on for Wii U hopefully - a handheld, 2.5D Metroid (or even something like Other M), or maybe a new Donkey Kong game like the one on N64.
I think they need some new IPs too.
But yeah, Smash Bros 3DS and Wii U may have footage, but I doubt anything will be playable, and I doubt we'll have many announcements for which characters will and won't make the cut (other than the obvious Mario, Link, Kirby, Starfox, Pikachu etc). We might see a new gameplay mechanic, or item (like the Smash Ball was in Brawl) but they'll likely keep all the new characters hidden for quite a while yet.