So my idea involves Castle not legally changing his name to Richard Edgar Castle as he did in canon, just adopting it as a pen name, but still using his real name of Richard Alexander Rogers on all legal forms and whatnot. The purpose of this? Because he's not just a bestselling author but many years before the start of the series he took and passed his tests to become a licensed P.I. and gun carrier, advertising himself as Dick Rogers, Private Investigator. While he continues to write his novels as Richard Castle, he's also taking on cases outside of law enforcement.
Hilariously enough, when murders start mimicking his early works, he gets brought in by Detective Kate Beckett for questioning and as it turns out, she reads his file as if Richard Castle is his real name. Rather than going along with it, he will correct them about his name and inform them that Richard Castle is just an alias; a pen name he uses to protect himself and his family. When he gives them his real name, they are shocked to discover that he has no criminal record under that name and that he's a licensed P.I. and gun carrier. This changes just about everything, since Rick's not an inexperienced writer but someone who specializing in investigating and solving mysteries, which is precisely what they are dealing with. It changes the dynamic of how they approach him for help and how he approaches them to help.
I don't know for certain if this is something I'd be able to write, though, but I thought it was a pretty good idea...