Anyways, Zelda has desperately needed some co-op features for a long time, even if it's sort of multiplayer with seperate things. Challenges, time trials, gauntlets, etc. Would add some medicore replayability, unlike the only replayability value being a hard mode (not that I don't want a hard mode, I love them. I would like a much more complex hard mode than the ones offered in Master Quest, Second Quest, and in Hero Mode). Personally, I think just leaving the main game to just single-player is the best option, and all that's needed is something like Halo: Reach's Firefight/GoW2's Horde. Mindless, easy to do, simple to get in to, yet fun.
Maybe if you beat the game, the seond time you play your Zelda instead of Link.
That would require them to practically make a nearly seperate game.