Well lets look at an MMO from the fee standpoint, even one made by Nintendo. The concept (whatever it would be) would have to revolve around a environment in which a, well, massive amount of people could play on. This results in the need for dedicated servers, moderators, and a development team that could update content / fix content should it need to be done. This would result in a financial hit to the company if they had no monthly subscription fee.
At the very least there would have to be some sort of revenue from the game itself for Nintendo to make the venture worth their while.
That and the lack of HD for the Wii and it's "lack of power" (in developers eyes) when compared to the Xbox 360 and PS3 makes it a less than ideal breeding ground for MMO type games outside of Nintendo.