I think the GamePad can be used as a game camera for non racers in MK8. In Smash, it can be used for stages with moving hazards.
For Mario Kart, maybe the gamepad can be used for people watching the other one to four players playing split screen on the TV. It can be used for changing the game camera which would only affect the gamepad screen not the TV screen. Perhaps the gamepad players cam also highlight routes on their screen which would affect the TV screen. This can be used to point out possible shortcuts.
In Smash Bros, the GamePad can be used to control stages. For example, think of a pong stage where there is a ground to battle between two paddles moving up and down. Players with the gamepad screen can move one control stick to move one paddle (left stick controls left paddle). The two playes will be playing an actual pong game with the ball bouncing back and forth. The ball would not physically push the players but will bounce off of them. After some time, if items are turned on, the pong ball may turn into a smash ball that is being bounced back and forth. The paddles will also physically stop the player from flying off stage and bounce them off like a regular pong game.
When the player is being bounced off the pong paddle, the force and momentum which the player is moving from the bounce is determined by their percentage. Player will probably need a red damage % to get bounced far enough to the hit the other paddle. Character weight will also be a factor. The paddle will not increase the character percentage, though. The opponent can also hit the bouncing player as well, but if mistimed, the opponent will get knocked by the bouncing player too. There can be a point where the gameplay players are not only bouncing the pong ball, but also the fighters! They can even have a score on top that records how many balls or fighters they scored.
Pong stage can look like this.
(Turns out there is a white line in the bottom fighters can use as platform. The top line will probably only affect the ball and won't stop the players from going off screen.)
