I think they will unveil Haeresis or the new KH series. If it is the later, I guess is it will be on PSP. It would make sense considering that the video from KH2:FM is on a PS2 game, and it would make sense that Square would use the KH2 engine to keep making money by making more KH games on PSP while they work on the PS3 one(s).
A while ago Sony had told third parties that the best way to generate enough revenue to finance the initial cost of next-gen development was to keep on making PS2 and PSP games to generate revenue until the PS3 was more firmly established.
And btw Square is
DEFINITELY planning a next-gen remake or sequel of FFVII. I am sure of this because of a few reasons: First there is Dirge of Cerberus and Before Crisis, setting the field for a sequel to FFVII. Then there is the fact that the team behind FF is saying that they don't want to make another Final Fantasy game that cannot be almost on par with Advent Children as far as action scenes/battles go. They said in interviews how AC raised the bar, and it would be disappointing to have fights that would not be on par with the movie (jumping around etc, and fighting on motorcycles). Imagine playing a sequel to FFVII, with Cloud, but with a battle system like Lost Odyssey. It would be wrong
So what are they doing? They are making FFXIII to build up the pipeline needed to make a new next-gen FFVII game happen. FFXIII has very dynamic battles that make use of an Endorphine-like animation system (mix of physics and AI to generate contextual animations that blend seamlessly with one another, as seen in White Knight Story for example, but crazier). They are trying to replicate the kinds of fights we saw in AC in preparation for the new FFVII, they are building the pipeline indirectly through the creation of FFXIII (and at the same time generating revenue from the game once it hits the shelves).
Add to this the fact that you can ride motorcycles and even fight while on motorcycles in FFXIII, and you get a clear picture of what they are aiming for. Expect a next-gen FFVII to be released after FFXIII, with fast-paced battles and the possibility to ride a motorcycle. Everything points in that direction