I very, very seriously doubt that it won't be Wii U compatible. In fact I wouldn't be shocked if it were also still Wii compatible. The main reason Wii U dropped GC compatibility, is because they didn't want to have the memory card and controller slots still on the machine.
At the VERY least, I think Wii U games will be playable on NX, because the NX will very likely still have some manner of Gamepad type controller set-up.







