The closest I've come to playing as a pre-existing, not-mine original character was someone that was already treated among her fans as being whatever-the-hell-you-want. The ongoing thread from whence she's from preaches a considerably laisez faire canon to any character, there's often some consistency between interpretation; but no one would give a shit if they're any different from an "accepted" personification.

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I am of course talking about /mlp/'s Satyrs.
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Otherwise there was that one time I adopted someone else's character(s) in Precipice. But I didn't really do anything as the one and we effectively decanonized the other.