Fan ideas aren't usually a viable option because of legal complications.
Teddy Antonio, the person who was credited for the story of Keep Calm and Flutter On, was a 15 year old fan of the show who happened to have connections in the animation industry (those connections being his parents, who are writers). The basic plot of the episode was already planned out, but Teddy got a chance to pitch ideas to McCarthy, who then passed his ideas, along with her own, onto Polsky to write the episode.
That's
probably the closest thing to a fan-pitched episode we're ever going to see. Episodes aren't based on fan ideas for roughly the same reason that the show staff aren't allowed to read fanfiction. The possible legal ramifications are just too great.
That being said, sometimes I do wish that the fans could have input into the episodes of the show.
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