I don't even see second game pads on sale in stores. When they do, it will be expensive. If they do release a game that supports two gamepads, it will likely have very simple implimentation. Nobody will take advantage. They'd be better off making a game that uses the 3DS insted. At least people will have more than one of those.

Which game is the question. I'll say Pokemon World.

One second thought, I'm gonna say Wii Play U.

Mario Party 10.

You heard it here folks

I don't see it happening to be honest ever.

Nintendo prides itself on being "mom friendly", and the last thing mom's want is two giant gamepads clogging up their living room. I remember Miyamoto taking great pride in that wireless controllers had solved the problem of cables all over the living room because it was a big complaint from mom's that they got from previous consoles. 

I think Nintendo would release a tablet-free Wii U before doing this (ala the way they dropped 3D for the 3DS).

Beyond that where would be the audience for this? The Wii U marketbase is already small enough, how many of them are going to buy a second game pad? Like maybe 5%? 

2nd Gamepad is not going to happen.

Had Wii U taken off, it surely would have, but its not and that would put too much risk in hardware costs to do.

Sorry, not worth guessing at a game that simply isn't going to materialize.