Hmm, not sure about animation, but Fanfiction.net lets you post whatever you want. Of course, none of it is legal, but it's a good place to refine your thoughts of such fan ideas before moving into much bigger stuff. Really, it's kind of where I started writing ideas like alternate universe for episodes of certain shows I loved and so on.
Truthfully, this story, Letter of Hope started as a fanfiction before I changed and removed a good portion of the copyright.