The Silicon atom is about .24 nm. Carbon is about .14 - a bit more than half as big. It looks like there's still some room to make transistors smaller than the sub-10 nm. Not much though. I don't see any solids with a diameter smaller than carbon so that may be it for the traditional binary logic building block.
It's getting close to time to look for something that will give a couple more states from the same size device.
Feynman said that there is plenty of room at the bottom. We are hitting the bottom and it is full of surprises. Graphene, CNT, Nanowires, variation of FinFET will compete for the lower technology nodes for sure.