Hmm. This is an interesting argument.
As stated before, Zelda isn't really meant to be nothing else besides a single player action/adventure game. The
FS series attempted to do it, and it did poorly. Not all games work out or need to be a MMO. Just look at the Elder Scroll's series and their success. Their new installment in the series is probably one of the greatest games of all time (this comes from more then just my personal perspective).
Anyways, multiplayer as a main gameplay element wouldn't work out. Now, you've already said this is not what you meant. Now, as a "bonus" in a Zelda games seems more probable, but the idea of it doesn't sound like it would be a very good. Firstly, Link is the main character. And you can only play as Link (besides
MM, but I don't think Nintendo will bring back the masks). It would be kind of hard to create fighting styles to other characters when you play ONLY as Link in the whole game.
Fighter style gameplay elements (i.e. Soul Caliber, etc.) won't mix with a action adventure game. You can have one or the other, but mixing them isn't such a good idea.
And as already said before, if you feel a need to play with different Zelda characters, and make them fight each other. Play a Super Smash Bros. game.