It would split the market. There would be Wii U and Wii U Plus games. The N64 did it, but only a few games required it, most just ran in an enhanced mode if it was available. Developers want to reach the largest market possible so they would probably make regular Wii U games and support an expansion as an after thought. Just like the Motion Plus. There are a few like Zelda SS, but most games don't bother with it.
I just don't see a Wii U Plus happening. I think we are going to see Wii U act much like the Wii did once the other next gen system are released. The PS4 and Xbox Next are going to be more powerfull and get the more complexe games. The Wii U will do fine because of Nintendo 1st party games, but 3rd party multiplatform games are going to be scaled down ports, like Modern Warfare 3.
