I'm not completely against the idea, but I think it would be better left for a spin off game than part of the main series. Zelda is all about the single player campaign and if you start adding a second player it could mess up the dungeon designs and puzzles. One way to counteract this would be to do something similar to Portal 2 and have a different campaign for Co-op. The story would still remain the same, but puzzles could be remastered to incorporate two people having to do stuff. I think that would bring some freshness to the series.