Yeah a Zelda MMO would be pretty cool. The other Nintendo franchise that would also make a great MMO is Pokémon. I could see hundreds of people as custom trainers in an open world, maybe several previous countries like Kanto, Johto, etc, and you could have battles, tournaments, leagues, etc. They might have to restructure it so you can't capture some Pokémon, like the one-of-a-kind legendary ones, but they could take a cue from the movies and maybe have a group go to New Island to have a raid against Mewtwo and his clone Pokémon.
Lots of possibilities there I think.