This can only mean Metroid.
Nintendo runs a very tight ship as far as leaks are concerned, and nowhere is this more evident than with fan favorite developer Retro Studios; the studio, which shot to fame for its successful Metroid and Donkey Kong reboots, has widely been expected to make a new Metroid game for the Wii U, but so far, all those expectations have not amounted to much, and all we have gotten form them has been the (admittedly excellent) Donkey kong Country: Tropical Freeze.
However, an insider who recently toured Retro Studios confirmed that the developer is working on a brand new Wii U game (with a possible E3 announcement?), and that they are using a new engine for this game. This is, of course, instantly a cause for interest, as it means their new game is not another Donkey Kong (or else they could have just used Tropical Freeze’s engine).
In that case, what exactly could this new game be? Might we be looking at, finally, a new Retro developed Metroid game? Nintendo has already confirmed a Prime style Metroid game in development for Wii U.
Whatever it is, we will keep you posted.