Rincewind the “Wizard” and the stage character of Groucho Marx.
Nah. He of course would try to assault and enslave her; she’d defend herself. Such a pity that she “misjudged her strength” and “accidentally” broke most of his bones and castrated him. Oh, well, these things happen. 
Dr. House & Dr. Quinn could be amusing.
Jubilee and Tonks would probably be a lot of fun.
And of course the team of CSI Vegas and Scooby Doo.
MacGyver and Steve Urkel
Urkel uses a bag of paper clips, some peanut butter, and a turkey baster to blow up the bad guy’s secret hideout.
“Did I do that?!”
Joyce Davenport (Hill Street Blues) and Jack McCoy (Law & Order).
Now, get back to work or I kill the little flying monkey!
Mr. Darcy (Pride and Prejudice) and Mr. Darcy ( Bridget Jones Diary) Colin Firth meet Colin Firth
Van Helsing The 2004 version. and Sherlock Holmes
Michael Corleone and Terry Benedict (Ocean’s 11)
Is that aimed at me, or does somebody else send out genetically engineered winged howler monkeys?
The manuscript’s in the hand of my pseudo-agent, so there’s no work to be done on THAT, and the family history awaits my client delivering the rest of the photos she wants printed. I 'spose I could look for some more clients, though.
Or play ping-pong. I loves me that ping-pong.
The Young Riders* and the crew of Serenity. I think they’d all get along great, actually. And if the Alliance showed up…well…say hi to a Nock gun. I’m sure Jayne would love one, and Mal and the Kid would hit it off. 
Recently introduced to this show and I love it now.
Oh, uh, okay. Never mind. What I have is a genetically engineered winged macaque. My mistake. Continue to not look for more clients.
The Young Riders was my favorite show when I was twelve and thirteen (it was my first introduction into “shippiness,” though it’d be years before I heard the term. Who wouldn’t want Lou and Kid together?) I’m afraid to get the DVDs since the magic could be gone.
They’d have fun with the crew of Serenity, for sure.
“My Mother the car” and “KITT”
I am sober.
Tommy Gavin and John Gage. I think Gavin would kick Gage’s arse on GP, but they might share a beer first
Sgt Schwartz
Willow Rosenberg and Dana Scully.
“An alien conspiracy to take over the Earth? Neat! Well, also very bad and displeasing, but also neat!”
Dr. Hannibal Lecter and Lord Voldemort
You’re SOOO passive-aggressive sometimes.
Sherlock Holmes and Spock
Launchpad McQuack and Goose (from Top Gun)
Gargamel and Lemuel Gulliver
Plus there’s always the hope of a tasteful shower scene with two hot redheads.
Two or more mutally incompatible versions of the Christian God. (Like the one from a Chick tract meets the one from Time Bandits, meets the one from some touchy-feely-glurgy Lifetime movie.)
Come to think of it—there are a lot of things you could crossover with a Lifetime movie and make something watchable. Child of Rage 2: Army of Darkness, anyone?
Roland and Doc Holliday.
Yea, I know Holliday isn’t fictional, it’d still be pretty cool.