Well, if there's any one franchise of Nintendo's that could definitely be turned into an amazing MMO, it'd be Zelda. All of the different character races make for a fantastic starting point to entry. I'd be on board -- monthly fee and all.
I could definitely see the world of Hylia being expanded on, too. There's just so much one could do with this franchise as an MMO. Same goes with Pokemon also.
Make it happen, Nintendo!