If the Wii U is a significant enough jump in graphics i think it will be more then just a direct port. Once a system is 2-3x the power of 360/ps3 it becomes effortless to upgrade textures, framerate, anti-aliasing, shadows and a few effects here and there.
Effortless? Upgrading the textures takes time. The biggest graphical problem with AC is always the horrible screen tearing so let's hope they'll fix that with the Wii U version