The Pokemon MMO always gets pulled out in these conversations as some sort of theoretical game changer, but I really don't see it. Pokemon may have as many, or hell, many MORE fans than Warcraft, but how many of them have credit cards, or the persuasive skills to convince someone who does to pony up $15 a month?
And ignoring that, Nintendo's never made a game with competent online, never mind something ridiculous like a MMO.