I think this is a total no-brainer. I have no idea why there isn't already one out there.
It's got all the hallmarks of existing MMOs:
Crafting - you can become something other than a fighter/trainer.
Hunting - catching and trading your finds.
Market - a rich trade in training, buying and selling, and of items crafted such as health and whatever else.
Exploration - the world is there already, well detailed. I'd love to see it in fully rendered and more 'realistic' visuals.
Questing - obviously, the biggest draw for many mmo players, the ability to get a goodie out of a travel/fight. Again the world is rich with this.
So... really, I can't understand why it hasn't been done before now. I honestly haven't played more than the very first handheld thing, and even then it was years after release and borrowed from a friend. I know more about it from the comics and cartoon angle, really, but it seems to me that the whole thing is just RIPE for an mmo.