i have to disagree with you. there isnt just hyrule, link, zelda, and ganondorf. you have termina, you have the places from oracle of ages and seasons, you have the place from links awakening, you have the twilight relm, the dark world and of coarse all the legends each territory holds. there is more than enough to build an mmo off of. matter of fact, it could even it perfectly in the "timeline". after ocarina of time, gannondorf return, and there is no link to stop him, but that dosnt mean everyone sat around and said "oh, well we have no hero of time to save our ass, so lets just let this big ol, bad guy with the triforce of power whipe us all out. im sure that gorons, zora, hylians, and even the kokiri, went to march in and try to kick some gannon ass!
anyway, the point im making by bringing up that portion of the "timeline" is that, really there is no way possible to make the most exciting part of the "timeline", a single player game, or not a game at all, so an mmo format would be perfect for that part of the story.