I don't know. But honestly, Kingdom Hearts 3 is very plausible. But I won't keep my hopes up.

From the words of the reveal:

"It is a ‘port’ but more of a complete rework for the Wii U: a bit like the work we did on Deus Ex: Human Revolution – Director’s Cut."

It gives the impression of being a port of an already launched game, because otherwise it wouldn't need to be reworked.

maybe the game isn't out yet but it's almost completed but they want to change something: 

First example that comes to my mind: Pokken Fighters: the game will be out this summer in arcade form, probably it's already completed and they started debugging. As of now the game in it's rooster (not fully unveiled) doesn't have Greninja (random Pokémon that suits such a game) but Nintendo, for waterver reason, wants to have Greninja in the game, so they have to change it...

It can be applyed to almost every single game...

Why do some think it will be KH3? Didn´t they say their port will release this year? If so it can´t be KH3...

OT: How bout the new NFS?

guys... It is not KH3. Unless its from Nintendo themselves, I doubt its even a Japanese game.

SE would never allow these guys to mess with their "pillar" IPs. They would do it themselves or have a a handpicked Japanese company theve worked with before do it.

Like Hexadrive, they do alot of SE contractual work.

Leaving aside the fact that Pokken Fighters is a bad example (either Nintendo or Bandai-Namco will make the port), the problem is that most "Top Notch Publishers" have internal teams that can deal with an extra version of a game that's in development because that helps with the comunication between teams and find and solve any problem.

At least that's my impression.

Your absolutely right on all fronts. If a port or contract work on a established happens it will be a Japanese developer that the contracter has a relationship with. 

How often has SE allowed a western developer to work on or port their Japanesey games?

At the moment I'm thinking it might be street fighter

Guys... I think we are missing a vital clue in our way to finding out what this mysterious game is! And that is this:

"We’re doing that. It hasn’t been announced, but it will be soon. And can I just further add to the mystique and say that it’s big. It’s the biggest thing we’ve ever done. And it’ll come out this year, and I hope it won’t disappoint." http://nintendoeverything.com/tantalus-wii-u-port-will-be-announced-soon-coming-out-this-year-studios-biggest-project-yet/

We have all assumed that the 2 first sentences refer to the port, but what if it is actually referring to the game that they are porting? If that is the case then we can just shorten the search to the small timeframe of February 26, 2015 till around the 1st week of June, apply the titles that have been revealed during that time to the criteria they gave, and we could have our answer!

Also... what is up with all of those KH3 predictions? No way the game is coming out this year, it would be impossible!