My guess is Star Fox.
Iwata tried to get Sakurai to do Star Fox or Kid Icarus and he took Kid Icarus. But it shows that he's been looking for a studio to reboot Star Fox.
Retro has had success bringing back Donkey Kong Country, now presumbly I think DKC has moved on to maybe NST in Seattle. But now Nintendo probably feels comfortable they can make more successful DKC games because Retro brought it back with a very high quality.
So perhaps Nintendo's thinking is Retro can make Star Fox relevant again too.