Well, as I said on the KH3D talk page, I think this secret ending IS in fact "3D." Why, you ask? Well...
1. The main characters of the game are Sora & Riku. In the secret ending, Sora & Riku are to take the Mark of Mastery exam.
2. In the game, this "Sora that we are not familiar with" could be that Sora is actually obsessed with winning, whereas Riku is not so competitive. Thus, Riku flies and Sora falls.
3. If Sora & Riku are to participate in the exam, they'll need to go to new worlds. Luckily, KH3D has just that.
4. The previous secret endings were for handheld games. KH was for 358/2 Days and KH2 was for BbS, right?
5. I don't think this is another data-based game like Kingdom Hearts coded. That would be like re-using the Card battle system from CoM. Hey, have you noticed they both had remakes?
6. By the time Re:Coded comes out in America and Europe, BBS Final Mix will be out in Japan... with another secret ending! With that, we'll have most, if not all the answers.
7. I don't think Sora & Riku's outfits in those screenshots from E3 were the real thing. It was probably just a technical demo to show off the 3DS's powers - hence "3D Demo." Besides, why would they give out so much info on a game for a console that's not even out yet?