I would love to see an episode like the Blue Butterfly, but its a 'what if'. As in, if Beckett was the mystery writer and Castle was the detective.
It could either start from where they first met, (like the pilot), or just be a follow on as in Kate already is at the Precinct and following Castle.
So in the first scene we could see Kate dressed in pajama's and a robe and she could be dancing orsomethingin the loft, which is 'Castle's loft but different furniture etc. Then she gets a call from her parents, who are both still alive, asking when they'll get to see her or something. And you find out that she is this bestselling author of Nikki Heat, (and if she's already following Castle Damien Storm). Then it goes to Castle, who lives in 'Beckett's' apartment, which looks different again, and Castle is eating breakfast when he gets a call about a murder. (But his name isn't Castle, its Rogers, cause he never had to change it). Alexis comes out and they talk about how she's at Colombia on scholarship and about her mother, Castle's wife, who we find out died just after she was born.
At that point we here Beckett's voice say, "hold up", or something, to which the shot cuts and we see the actual Beckett and Castle in bed. Castle has actually started musing about what would be different if they had reversed roles, being the writer he is. Beckett starts already poking holes in his story, saying obvious things such as who says they ever would have met and that 'alternateuniverse's' are so unpredictable.
Anyway, the whole episode is a case done with the roles reversed and Beckett and Castle arguing about it in bed together. And either itsBeckettand Castle's first encounter or just a case they are working on.Montgomerywould be back in charge, and Ryan and Esposito work with Castle. Martha is still an actress, but she's successful, and keeps wanting to give Castle money and things but he refuses. Beckett has never been married but never manages to stay with a guy long enough. And we find out that Castle's wife died while he was starting as a cop and that's why he went into homicide. I'm not sure if Lanie would be the ME still, or whether she would be Beckett's publisher or something else. (That would make a great cut scene for the real Beckett to tell Castle that Lanie's life wouldn't have changed if hers did and poke more holes in his story).
*I may have thought about this a bit too much. But I think it would be a nice, fun episode, and different perspective*