As you already know, Retro Studios has been working on a game since late 2013, after they completed DKC: TP. Ever since then, i've been thinking of what it could be. Recently, Retro hinted that it would be revealing its next game at this years E3, which is right around the corner! Now, I feel, is a better time than ever to guess what it could be! Some possibilities would be:
A new DK game - Only in 3D, like DK 64. This could very well happen since Nintendo is aware that a third 2.5D sidescrolling game is something not many are interested in anymore. A 3D DK could have a lot more going for it.
A new Metroid - This is my dream. While it's not far from possible, it's certainly something that I personally don't have much hope in. We heard rumors of Retro working on a new engine and all, but that could totally end up going towards something else (see above). Though, in my humble opinion, if any studio knows 3D Metroid, it's Retro.
A franchise revival - So far, Nintendo has given Retro two franchises to revive. And both of them are fantastic! So, this could be a likely scenario where Retro is tasked to revive an old franchise. What could it be?
A new IP - There is also the possibility of them working on something completely new. Though, I find this the most unlikely of all my guesses.
So, those are my guesses. Anyone else have any?