It will not be Mario or Pokemon related
Nintendo is apparently gearing up to launch a free-to-play game this fiscal year (which ends March 2014), as announced at E3 during an analyst briefing. Evidently the game has been ruled out as a Mario or Pokemon game, and it could be anything from an already existing franchise to a brand new IP.
A platform isn't set, so it could be anything from Wii U or 3DS. As I heard this news, I had a thought -- if done right, this would be an amazing way to re-introduce the Advance Wars series to a new audience. Although "free-to-play" generally has a negative connotation, it can be done right, and given how well and sparingly Nintendo's handled DLC so far I trust them to at least try in earnest to make it an enjoyable experience.
Nintendo to launch [Nintendo Life]