The best advice for how to write good fanfiction for MLP or anything else is don't be a fan. I don't care how many episodes you can quote from or what your headcanon is; just tell me an original story.

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Undead Robot Bug Crusaders: https://www.fimfiction.net/story/268833/undead-robot-bug-crusaders

My fanfiction: https://www.fimfiction.net/user/KATCompositions