Imho the only kind of Multiplayer I can think of is if they add some sort of EXTREMELY strong boss battle that to be defeated you either have to grind all of your weapons almost to the maximum or team up with 1/2 friends, tough it must be after the end of the story because I don't know how they could explain it otherwise (unless some sort of time-crush event where every player is a different age of Link but everyone sees themselves as the Link from the main story, or again an enemie that can use magic manages to split link in the 3/4 Links...)
Oh, another thing I was thinking of recently is that to solve the problem of the "empty overworld" they could make so that in every place that isn't a dungeon/village/place related to story a certain number of your friends can appear and act along the lines of merchants that'll give you different weapons, clothing etc...
