IGN.com has a quote from ABC President Paul Lee that gives more details. According to IGN, Lee said, "You know, I don't know if I'm allowed to say it, but we've got a really cool storyline with a Marvel character that appears on Castle this year." Lee went on to clarify that the Marvel character would be a Marvel superhero, who would appear in a Halloween episode.
Considering that Nathan Fillion is a huge comic book fan, this crossover should be quite entertaining. Since it's a Halloween episode, odds are it will just be a regular person dressed up like a Marvel superhero, but who knows, it could wind up being an actual crossover with a new series in development by ABC. ABC has pilots in development for shows featuring Marvel Comics characters Jessica Jones and The Incredible Hulk.
As we previously reported, there is also a Castle comic book in the works. Castle: Richard Castle’s Deadly Storm is being co-written by Brian Michael Bendis and Kelly Sue DeConnick with interior art by Lan Medina and cover art by Carlo Pagulayan.