I have a variety of "options" for the Gamepad usage in a Wii U Mario Party, and I stand by these ideas mostly confidently.
There are 2 main ways of applying the Gamepad's usage. It can be used by actual players, that is, actually playing on the board and in minigames as a character; the other way is to have an unrelated neutral player. This neutral player (a human) will be given the Gamepad and can affect a variety of computer-controlled, luck-based events. These include affecting dice block roll, minigame selection, Orb choices, etc. on the board, and things in minigames such as which mushroom would be safe in Mushroom Mix-Up. This would be very interesting because many people complain that luck is too prevalent in Mario Party and now with someone actually greatly affecting the "luck", it could be more controllable.
The other way of using the Gamepad is by giving it to the actual players. The first way of using it is by assigning it to specific players; this can help close the skill gap between skilled players and new players, as the Gamepad could be assigned to a new player and would give them an advantage on the board or during minigames. These advantages include actual changes, such as more lives in a minigame, to inherent control advantages, like the Touch Screen being easier to control as compared to pointing at the screen. The other way to use the Gamepad is to treat it as an "earned privilege". Here, you can assign the use of the Gamepad to a certain place. For example, the 1st place person has earned the Gamepad, or maybe the last person needs the help. Of course, it can be changed to random so a random player gets the Gamepad each turn and/or minigame.
The other way of using the Gamepad as a player is by giving it to an extra player; that is, a 5th person. This revolutionizes the gameplay in that there can now be 5 people playing, with each minigame affected with detail by how many people are playing. Like in Mario Party 9, the number of players per party could be 2, 3, 4, or 5, so using the Gamepad in this manner with less than 5 people would actually be like assigning it to a specific person mentioned earlier. So now, 4-P minigames are Free-for-All minigames, 1-vs-3 minigames are 1-vs-Rivals minigames, 2-vs-2 minigames are Cooperation minigames. Duel and Battle minigames do still return.
In conjunction with the "earned privilege" Gamepad, it is possible that if a 1-vs-Rivals minigame pops up, the lone player always gets the Gamepad. This is changeable in the Party Options. Next, if there are an odd number of players, the Cooperation minigames must all be changed to factor this in; in MPDS' Airbrushers, the third person provides more area for the team to collect coins, but the extra weight means it's harder to fly upward.
In the options, it is possible to choose which way of play the Gamepad is used (neutral player, actual player, privilege), whether to limit its use to the board, minigames, or both, or to turn it off completely.
8-P mode also returns, although it can be changed to become 6-P mode (3 teams) or 10-P mode (a team always uses the Gamepad).
I'm not quite sure if these ideas make sense, but if you have questions, ask away.