Yeah they should stay away from backseat driver kinda multiplayer.
If they should keep the camera like recent Zeldas (Ocarina of Time, Majora's Mask, Wind Waker, Twilight Princess, Skyward Sword) then surely they won't be able to have 2 player on screen multiplayer. Unless it's splitscreen. But I'd rather they'd NOT have such multiplayer. Perhaps one person could play on the TV while the other play on the Wii U Pad. But still they'd have to two different renderings this way and it costs more cpu/gpu to do this. Which means less good graphics and probably less enemies on screen and similar. This could be done via online-only-multiplayer aswell but that kind takes away a bit of the immersion, unless you have good ways to communicate with your friend.
However, if they went back to old Zelda's style (Zelda 1,3,4,Oracles, etc.) they'd be able to keep two or more characters on the screen at the same time no problem.
Imagine playing Link, a Zora, a Goron and perhaps Zelda herself! Where they all have different skills and abilities they can gain.
Then again they're free to introduce new characters to the series aswell.