I was just thinking that it would be cool if the show touch on the basis that some murder cases were still unsolved after Castle left Beckett for the summer. Beckett is a good detective but she would not be able to solve some of them without Castle.  Now that Castle is back, it would stress that Beckett needed his "creative imagination." It would implied that she needed him. Castle helps her solve an unsolved case and Beckett admits to herself (out loud) that she needed him. Castle overhears her comment without her knowledge and just smiles.

Also, if they could include a scene where Beckett is trying to discuss a murder case with someone else (like her upcoming SO) but the person couldn't show the same passion/interest/excitement as her discussion with Castle. Frustrated, she begins reflecting and longing for her "discussion" with Castle.