If they did that I would demand that they also include a green hat that I can wear while playing. The hat wouldn't do anything gameplay wise, but by that point I think they should just go all out. Plus, it'd look hilarious.
I also hope that they keep the Skyward Sword controls.
Using the Gamepad as a shield isn't really feasible though. How are you supposed to comfortably control Link while holding the Gamepad like a shield.
I hope they do use the Gamepad though. It can be off to the side doing something like menus and maps and inventory stuff. Or they could have it off to the side doing something relating to the games obligatory sidekick.
I've been saying something like this for a while too.
Refine the motion controls a bit, of course. Take into consideration the things that people liked or didn't like. Add in some controls for more special moves like back slash and helm splitter and maybe something new.
And beyond that? Maybe use the gamepad as sort of a Tingle Tuner type of interface. See an overhead view on the gamepad screen. Hit a button and the screen shows you the world from your fairy's perspective (with points of interest that Link can't see himself lit up). Tap some other buttons to activate some simple helper spells (like Tingle's items that he used on the tuner in TWW). Maybe add some touchscreen funcctionality to them. Move the fairy around to scout ahead. Having a second player to control the fairy for you would be nice but completely optional.
Or they could try to go all out with the co-op play (although I personally would really hope that it's optional and doesn't affect single player) in some way inspired by the Zelda-themed game in Nintendo Land? Play as Link (sword and other melee weapons/tools) and Zelda (ranged and magic) or Link and any other possible secondary hero. Give both players freedom of movement instead of working on rails, of course. Maybe the characters can share an inventory but the items control different and have different effects for each character. And in single player mode maybe there should be an option in the pause menu to tell the game that you're switching controllers (so you can switch over to the other player) and the one not being controlled will have enough of an AI to get you through teamwork-oriented puzzles.
"This ain't like fieldwork. You never have to notarize a man and kill him in triplicate. Well... Almost never..." - Ruby