I had this idea involving Princess Celestia and the rest of the alicorn princesses facing a secret cabal of ponies that wish to her and the rest of the royal families dead. It's not the most original idea and I know that there is a series involving Twilight being assassinated, but my knowledge on that fic is quite limited. And for what little I do know, I'd rather not rely too much on.

Yes... actually I have. That happens in a few of my fanfics in fact.

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If you want it, go for it.
Almost everything has been written somehow anyway.

5878086 A valid point. But I don't really like to be accused of stealing ideas. But you are right when you say everything has been done before. I guess it really all depends on what I can bring to the idea.