Last November Nintendo's CEO announced that their intellectual properties could be used by fans for derivative works (including for monetary gain). Backing the Zinetendo comes with zero legal risks. Here's a link to an article that summarizes the announcement: http://www.gamnesia.com/news/nintendo-properties-can-officially-be-used-for-fan-projects#.VSlnUGZZXMO
Commissions cannot be excessively violent or sexual in nature. You may choose up to two characters, but you aren't limited to those created by Nintendo. Original characters and those from comics/movies/television (and so on) are also welcome.
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