Who knows really too ... mobile/portable chips are starting to getting really powerful.
The Tegra X1 being released in a few months can run the Unreal Engine 4 Elemental Demo that was used to demo the PS4/XBox One:
https://www.youtube.com/watch?v=q0d3rFALqLo
This is a chip that will be used in mini-tablets and phones even.
The main issue is the memory bandwidth of mobile RAM versus home consoles, but I think Nintendo can get around that with eDRAM, even the 3DS has eDRAM (New 3DS has 10MB, perhaps a Fusion handheld could have 32MB?).
You could have a handheld even by next year that could run some PS4 ports, scaled down in resolution with some details trimmed ... and then a home Nintendo variant that maybe is 2x-3x that which can run the same port at 1080p at home with more effects.
