What is Retro Studios working on?

It has to be an existing Nintendo IP i feel. Starfox or Icarus are the only ones that jump out at me that could be possible. We know its not metroid and according to Matt IGN they were working on a Zelda but are currently not anymore. I WANT TO KNOW!

If Retro can do to Kid Icarus or Starfox, what they did with Metroid and the Prime series...

The thought is too good to process.