I'm noticing something really odd here. A few year ago, everyone was for Star Fox or F-zero getting a revival. Now with DKCR and other events, it has split between Star Fox, F-zero, Donkey Kong, Zelda, Metroid and new IP. F-zero can be ruled out since it's obviously not in development and Zelda isn't happening since Retro has been hinting at a project everyone wants them to do since pre-E3 2011. I also doubt Donkey Kong since it's more or less already gotten it's revival and the cries for a sequel is not that high and Metroid, while many like, is nowhere near as wanted as Star Fox or a new IP and I doubt Retro is interested in a fourth Metroid game (the only reason why I would bother tracking Metroid Prime 4 is for the hopes that Dark Samus returns and can gain a strong fan base again).
I think they're working on either Star Fox or a new IP. Right now, I am saying new IP because as Starphoenix said, the art team doesn't look like the one to work on Star Fox.