100% rewards would be cool, but I'd really hope that it isn't dumb. In the past, when you get 100% in certain games, all you'd get was a cool ending scene (like the Metroid Prime Trilogy). Don't get me wrong, I love that it adds some SUPER important stuff for the story (origins of characters, bosses, stuff like that). But I don't think that Zelda needs something like that because most of the games are completely spread apart time-line wise.
If they were to successfully put a 100% completion in Zelda, it would have to be done so well so that the fans aren't missing anything from the story by not getting 100% and if you do get 100%, they could implement a new dungeon, or a harder play through for the next time. If the game designers were going to do something like that, I'd be all for it. But they have to be careful to not put it in just for fun. It has to work well.