I can’t access that thread for some reason, but it hardly matters. In fan fiction there’s no such thing as a crossover that’ll never happen, just ones that haven’t been done yet.
When I was younger and there were a bunch of video games based on crossovers like “Aliens vs. Predator” and “Robocop vs. the Terminator” there was one game I would have liked to have seen:
“Enterprise Security Team vs. Imperial Stormtroopers”
Will & Grace are exterminated by the Daleks
Charlie’s Angels get eaten by Godzilla
Friends in The Twilight Zone
Have you ever searched the crossover sections at fanfiction.net? You’ll find crossovers you never dreamed were possible. For instance, imagine what would happen if Luke Skywalker met… the Care Bears. You can find out here.
Oh, yeah. I might as well plug my own stories while I’m at it. I have three stories at fanfiction.net, and all of them are crossovers. There’s Synchronicity, a crossover between Star Trek: TNG and Doctor Who; Demon City DC, a crossover between Devil Hunter Yohko and the X-Files; and Samantha, a crossover between the X-Files and the movie Anastasia.
Back in the mid-80’s I went to KC Con in Kansas City Missouri. I was a Dr. Who fan, dressed as Tom Baker, and a friend was a Galactic Warrior, from BSG. So we worked out a little skit for the costume contest, in which the Tardis accidentally materializes on the battlestar, and I was confronted by it’s security. I couldn’t talk my way out of it and was led off to the brig, with my hand tied behind my back by my own scarf.
All in the Family and The Simpsons.
…can I reccomend watching this movie then, from Spacebattles.com …
http://spacebattles.com/movies.php?action=view&movieid=18
Enjoy! 
http://blackraptor.hispeed.com/naafic/notagain.htm
A crossover between Now and Again and Quantum Leap
Enjoy.
I’m still ticked off at those idiots at CBS for cancelling Now and Again.
For those in need of their Now and Again fix, you can read some fanfics here:
http://www.starpulse.com/Television/Now_and_Again/Fan_Fiction/
Personally, I would just like to see Captain Marvel (Genis-vell) paste Superman.
Jay and Silent Bob
Wayne and Garth
Bill and Ted
Two words: Gilligan’s Leap
I’ll go with a crossover of the original Battlestar Galactica and Knight Rider where it comes out that KITT is the original ancestor of the Cylons (that whole red sweeping eye thing).
I’d like to see a crossover of Will and Grace and Batman. The Batman side of it should be played straight, so to speak, and with the character in the style of the animated series. Heck, if Kevin Conroy is buff enuff, get him to play Batsy. Will, Grace, Jack, et. al. will naturally be lust struck by the crime-busting beefcake. Hilarity ensues.
Did Scully and Mulder ever investigate anything relating to the Cthulhu Mythos? I can’t seem to remember them doing so, and that woulda been a nice storyline.
Professor Sam Beckett, working from a secret government facility in Nevada, using only cocoanuts, developed a machine that would allow him to time-travel within his own life.
You know this would end up with Jack in a Robin costume.
I have a story plotted out, but only half-written, featuring a crossover between “Dr Who” and “the Avengers.” Synopsis: the Doctor (#4) & Romana investigate a time disturbance originating from swinging 1960s London. At the same time, Steed & Mrs. Emma Peel are investigating the disappearance of several prominent politicians. Both problems are caused by the Master using a time tunnel in an elaborate scheme. Steed & Romana get sucked through the tunnel to another planet, leaving the Doctor & Emma to chase after them in the TARDIS.
Other crossover ideas I’ve dreamed up (but never set pen to paper over) include the “Lost in Space” saucer being discovered by the Enterprise (the original Robinson family having long since died out and replaced by inbred descendants), “James Bond gets it on with Charlie’s Angels” and “the Munsters vs. the Addams Family.”
And just for the heck of it, imaging Gargamel the inept siccing Cthulhu on the Smurfs.
I’d love to see Hannibal Lecter drop in on the gang from Seinfeld. Unless he’d given up rude people for Lent then he wouldn’t need to feed again for years.
I always wanted to see James Bond arrested and confined to the island from The Prisoner. (This could also explain why there’s always a new James Bond every few years- the name is just recycled and given to a “younger model” while the old one is debriefed in a rather unusual way, plus I’d love to hear Sean Connery say “I am not a number, Trebek!”).
The first crossover I ever wanted to see- and this really dates me- was for Sanford & Son to pull their truck into Chico & the Man’s garage. I also wanted to see J.R. Ewing and Angela Channing (Falcon Crest) take on Alexis Carrington.