It's impossible. The Wii U is already maxed out. Scaling? Rendering at a lower resolution? Less anisotropic filter? Less antialiasing? Lower resolution and compressed textures? Lower resolution shadows? Lower resolution lighting? Loading less assets at a time? Nah, that cannot be done. This is not a PC to PS360 we're talking about. We're talking 'bout Nintendo, and when it's Nintendo, those things cannot be done. Trust me, I post at Neogaf, that makes me a pro.
endofsarcasm
Now all jokes aside, it's not that the Wii U won't be able to do the same games. It's about many developers won't bother doing it, and not because of the hardware, but because of the perceived market of the console. They think their ports won't sell, and won't even try.