Normally I'd say a Kirby stage; that's what I've been predicting for a few days, since we already have stages from Mario, Zelda, Star Fox, Yoshi's Island, Animal Crossing, Donkey Kong, Kid Icarus, and Fire Emblem. That leaves Pokemon, Metroid, and Kirby as some major first-party franchises--and Pokemon and Kirby already have semi-revealed stages, so I'd go with one of them. Of the two, I think Kirby's more likely because his profile has been basically the only SSBB update his franchise has gotten; Pokemon has two characters and two Poke Balls already.
But... this update will be on Kojima's birthday. So I'm going with a Metal Gear Solid stage. If it's not that, it'll be music, again, just to screw with all of our minds in true MGS fashion. I certainly don't anticipate Snake's profile or any sort of MGS-related item.