Star Wars and Star Trek.
Which could be a glorious trainwreck.
My pet theory: The Q (From Star Trek) are Force Users that become one with The Force and the Q character played by John de Lancie was grooming Picard to joining them.
I would love to see a crossover that explains how The Matrix is set in the distant future of The Terminator universe
How about the Avengers and The Prisoner; Steed and Mrs. Peel get taken to the Village and easily unravel its secrets while looking fabulous.
Maybe a Rockford Files/Moonlighting crossover. David Addison would make a good foil for Jim.
I forgot all about Sleepy Hollow, and I loved that show!
Maybe a crossover with Once Upon a Time?
I always hoped that the Patrick O’Brian Aubrey/Maturin novels would have some of Jane Austen’s naval characters from Persuasion show up, but AFAICT they never did. I don’t know if any filmed versions of A/M have seized that missed opportunity.
Also, as I’ve said before, I really want to see Jackie Chan and Shah Rukh Khan co-starring in a Hong Kong/Bollywood style mashup action thriller comedy.
I suppose Sharpe and Hornblower would work as a British Army/Navy crossover during the Napoleonic Wars.
For added fun, throw in Jonathan Strange and Mr Norrell.
I’d like to see a crossover between The Closer and Reno 911. I can just see Detective Sanchez blowing up at the goofballs in Reno, and the damage in L.A. when the Reno 911 crew arrives with all of their antics. Hilarity ensues.
Kung Fu Hustle needs to be mixed with something. But I’m just not sure what.
Seeing the landlady lay a beatdown on some unsuspecting slob would be hilarious.
Ideas?
I’d like to see a crossover between Buffy Summers at Sunnydale High School and Veronica Mars of rival Neptune High.
And throw in Sabrina. And maybe Riverdale if you want it to get really weird.
The X-Files and ALF.
Or Kirk and Vetinari have to deal with each other. (I wouldn’t be at all surprised if the latter has a plan…) And Vimes would meet a redshirt and figure out how to save him or her. Or if he couldn’t, he’d make sure justice was meted out…
That’s be a delight.
Of course, I’d love to see Kirk and Miles Vorkosigan go head to head, too.
Lol, forgot that I wrote a short scene…
Downton: Predator
Vorkosigan meets Vetinari sounds like fun. 
I think a Veronica Mars/IZombie crossover would’ve been delightful.
Geez, I would pay good money to see this. Comedy or drama?
Since I just watched this and kept confusing the titles, Reservation Dogs vs Reservoir Dogs.
Okay, I just thought of a good one: The Walking Dead meets Z-Nation.
TWD guys are being all angsty about how “humans are the real monsters” while screwing up all the time, and run across this band of people who are just having as much fun as they can while trying to save the world.
Doc keeps offering Rick a joint, and Rick says, “You know I used to be a cop, right?” and Doc says, “Hey, I don’t hold that against you!”
The part of me that loves absurdism would have loved to cross it with Bill and Ted’s Excellent Adventure, prior to the most recent movie. 
The thought of seeing some historical figure get in the landlady’s face and getting an epic beatdown while the duo said “Excellent!” would have been worth the price of admission.
Or you can cross it with any series that takes itself just a bit too seriously, and/or leans too much into the Grimdark. For example, The Walking Dead - the survivors rush to a separate living block of apartments, strangely untouched by the surrounding sprawl, only to find out WHY the undead give it such a distance!
Miami Vice and The Andy Griffith Show. Crockett, Tubbs, Taylor, and Fife fighting vice on the mean streets of Miami.
Otis would be a coke dealer, Gomer would run a car theft ring, Howard would be a numbers runner, Aunt Bee would be a street hooker, and Floyd “the Razor” would be her pimp.
The theme song would have Miami Vice’s melody, but whistled.