Based on Shikamaru Ninja's information that it will not be a racing game, and also based on his statement that a game developed for Nintendo is considered first party even if it's developed externally, along with the rest of the information he has so graciously given us about this mystery title, here is my conclusion:
Battalion Wars 3 for Wii U developed by Headstrong, perhaps with some help from another developer or possibly an internal team within Nintendo to make sure the game shines and is able to improve upon the first two.
This may or may not release along side a new Advance Wars for 3DS with some kind of connectivity between the 2 games.
If you think about it, it makes sense. We know Nintendo has wanted to continue exploring this franchise (there was a medieval spinoff in development at Headstrong, at Nintendo's request, before they shifted gears to the whole casual thing) and we also agree Wii U has a lot of potential for Strategy games. Add to that the fact that it is a fairly western-oriented franchise that has so far failed to achieve mainstream success and it would appear like a good fit.
To be honest I really wanted to like BWii but there were several things holding it back from greatness, not the least of which was online communication. Still, the fundamental idea of adapting Advance Wars into a console RTS is a good one, and many of the ideas already seen in the first two games are good enough to be keepers. You can also tell by the production quality that this is a franchise Nintendo want to see grow.
My $0.02
