Another I'd like to see:-
A victim has a rare blood condition which Ducky would like a second opinion. It just so happens that his old Haematology Professor is in town at a seminar. He asks Gibbs if he can get him to take a look. Gibbs says OK.
Now picture the scene:- The phone rings, Gibbs answers (in close up). He says OK, puts the phone down and tells everyone that Duckys Professor has arrived. (Pan back sharply) The Professor is standing right next to Gibbs and introduces himself. During the whole episode you never see him come or go, he's either there or not there. He creeps everybody out except Ducky, who's oblivious to the effect, and Abby who thinks he's totally charming for an old guy.
A later scene would be Abby coming in to work in the early hours. A long shot shows her totally alone. She sorts out her stuff, puts on her Lab coat and peeks into her microscope. She then starts talking technical, finally looking up and saying something like, "...don't you agree Professor?" The Professor, standing right beside her, takes a look into the microscope and concurs adding a few things she missed. Gibbs enters.
Abby "Hi Gibbs!"
Gibbs "Who are you talking to?"
A long shot shows the two of them totally alone in the lab. Abby looks around nonplussed.
Abby "The... Professor... was... right here!"
Gibbs "Do you have something for me Abbs?" and the scene moves on from there.
Special Guest Star:- Christopher Lee. (Although Udo Kier would do, or Hector Elizondo, or maybe Armin Rhuber Stall.)