The actor told E! Online that he is currently hoping to secure Kyle MacLachlan for the show.
"I just found out that Kyle MacLachlan is available," he said. "When I was doing Desperate Housewives I was stuck to that guy like glue.
"He is incredibly entertaining and incredibly talented. I really, really want to have him on Castle like you wouldn't believe."
Fillion added that he has written down the names of people he wants to join the show on his phone.
"I have a list of people I want to see on Castle," he explained. "Here we go. People to have on Castle: Kevin Sorbo, Sarah Paulson, Lyndsy Fonseca, James Marsters, Bo Derek, William Shat, Kyle MacLachlan - he's on my list - Michael Hitchcock, James Denton, Mircea Monroe, Alexis Denisof, Alan Tudyk, Ron Glass, I have the whole cast of Firefly here, Dylan Baker, Jaclyn Smith."
He added: "These are just some of the people. The list grows longer all the time."
Castle returns for a new season on September 20 on ABC.