The entire Actorverse. Who could have predicted that an RP about the Blazblue cast actually being the actors of the story would take off as well as it did?
Azure Days Rachel has been well-received for being kinder and more down-to-earth than her canonical counterpart, while still retaining her sense of class, wit, and sarcasm.
In the Ragna x Makoto RP "Cape Lilies", at one point, Makoto meets a talking koi called Codswallop - who is constantly angry and yelling at everything and everyone. This crazy carp has become a favorite among forumers.