Well their criteria takes out a lot of choices:

- No M: So we get lowered to E, E10+, and T rating as possible games

- AAA game: So no indies.

- Biggest Project Ever: We can throw out shovelware and small series out of this. Something with a E rating could also get thrown out the window, since aside some of Nintendo's games, E-rated games are not that big.

- From a top notch publisher: Well this makes it tricky, since the majority of developers publish their own games, though some like Square Enix are also known as publishers for other companies, like them being the publisher for CoD in Japan, and by what rumors say, are publishing Street Fighter V.

- Not "hardcore": Was that specificially what they say? Or was it more around the levels of them meaning not "hardcore" like Deus Ex and Mass Effect 3?

- A port that sounds like a complete rework of the game for Wii U.

- Coming out this year: So no FFXV or KH3 

To say the true, this criteria only takes my mind to one thing, and one thing alone, the Kingdom Hearts ReMIXes, it fits all the bills, well depending on what they meant about "not hardcore", it is not a M rated game, it is a "AAA" game, big projects since it is 2 collections of games, from a top-notch publisher(Square-Enix), and any game would fit the rework criteria, since the game would had to be rework because of the Gamepad, but there are two little things that make that rework part to make more sense for the collections if they ever hit Wii U, and those two little things are:

Kingdom Hearts 358/2 Days and Kingdom Hearts Re:Coded

The two KH DS games were putted in the collections just as movies, because the PS3 didn't had a way to recreate the touch screen parts of the games, and reworking KH 358/2 Days to work in PS3 would had taken around a year or more by what Square Enix said, now the Tantalus port has been going on for quite awhile, the rumor began in 2012, but the development was said to have began in January 2014, well for a "AAA next-gen title" though it is unknown if it is this, but the case is, that a year has passed and that would be enough to recreate KH 358/2 Days, then they would just have to recreate some parts of KH Re: Coded, since they can just use the models of the other games for it.

But the most important thing suggesting it could be Kingdom Hearts, is that Nomura promised us 1 KH game per year, and it is already March, and we do not know of any KH games coming out this year, and Square Enix usually reveals it 1 year before release, but this time nothing, but as Nintendo is connected to this, and they either reveal games way before release or just some months away from release, then that changes things a lot.

If it weren't for Microsoft owning the IP, i'd say you were right. It fits the bill in several respects, and i'm very suspicious that any other third party would give Wii U the time of day at this point: none of their games fit the environment on the console.

I think it will be either Final Fantasy 15, Kingdom Hearts 3 or a game yet to be revealed for the first time.

But there's never been a Kingdom Hearts game on a Nintendo console. The Kingdom Hearts fanbase has been conditioned to own a Sony console and a Nintendo handheld. There really shouldn't be any Kingdom Hearts fans owning only a Nintendo console..

I think you're going to be pretty dissapointed. 

This is an awful lot like when people thought Ubi Soft's "mystery" Wii U title that were sitting on was Zombi U-2 ... turned out to be some Party With Friends game.