2018 is way too late. They've been talking about NX since spring 2015 now, they're not just going to wait 3 years.
I say 2016 or 2017.
3DS needs a successor here soon too, it's over 4 years old now, and coming up on 5 years with declining sales every year to the point where Nintendo is now down to their lowest handheld shipments in 18 years. It needs to be replaced.







