To me this idea seems very vague. There seems to be no support behind it, meaning what will the criteria be. Picking ninja of the week can easily be manipulated and I don't really see a way to get through that.
Unless there is a "Random Ninja of the Month", there really is no way that this could work. With the exception of voting for your favourite person, like suggested; although that would most likely be ridiculous considering the amount of people that frequent the game.
While I like the idea of a "Ninja of the
Month", I doubt that it will make a serious, long-lasting impact in the gameplay department... Unless there is a very substantial reward.