It would also have been cool for it to crossover with Fringe as they are both regarded as being quite similar.

Doctor Who

It's fun to think about the possibility. Bones would be fun, except it didn't come around until after The X Files had already ended. I know one is supernatural and the other is more police procedural, but it didn't stop the cross over from Sleepy Hollow.

The Golden Girls.

It did happen in 1999. Lance Henriksen guest starred as Frank Black on the X-Files on a late 1999 episode named "Millennium".

millennium is still chris carters best show
Well,Dreaming costs nothing,but please don't make ridiculous affirmations like that.

IT DOESNT MATTER WHAT YOU THINK
millennium is still chris carters best show
spectre can
suck it.

"It's a shame this didn't happen with XF and Millennium."
I agree completely especially since Carter acknowledged that both shows took place in the same universe.

I was mindful that Twin Peaks ended before The X Files began however it was more of a fantasy 'what if' scenario rather than it actually being a possibility when both shows were on the air at the same time. It would have been cool! I don't know if it would work as well with how the x files is now. I think it became (and is now) too glossy and mainstream compared to it's dark late night cult vibe it had in its early years, much like TP.
I know that the characters in CSI did crossover to one another's shows on occasion. It's a shame this didn't happen with XF and Millennium.

You're referring to the Twin Peaks revival I assume? The original run of Twin Peaks ended in 1991 which was 2 years before X-Files premiered so no crossover was possible.
And yes, the "Millennium" episode from season 7 was essentially the series finale for that show.

Twin Peaks would have been a good one. I think that the Vancouver seasons had the same atmosphere so it would have been interesting to see.
I think they also missed a trick with not crossing it over with Millennium more often. I know they did the season 7 episode but I think this was partly a wrap up for Frank Black after his season 3.

parks and rec, ron swanson telling them their department is a waste of money and andy ironically being the one that helps them solve a murder

From what I've read,
Picket Fences
still had a character mention two FBI agents (not by name) investigating in a nearby town.
"I used to be a detective like you, but then I took a bullet to the knee."-Law & Order: Dragon Squad

Back during season 2 there
were
plans for a crossover with
Picket Fences

• the episode "Red Museum" was originally conceived for that purpose (note that it takes place in Wisconsin). After the idea was nixed (not sure why), Chris Carter took the story in a different direction, which might explain why it feels a bit all over the place. I'm actually kind of glad they didn't do the crossover - I like the episode a lot the way it is.
  If they hadn't killed off Donnie Pfaster in "Orison" they could have made him the subject of a crossover story with
  Law & Order: Special Victims Unit.
  

IT DOESNT MATTER WHAT YOU THINK
plus its set years before mulder and scully were partnered up.
spectre can
suck it.

I understand you think it's derivative, but to say it's leeching is more than a little harsh, IMO, especially since TXF didn't do its think until 1993, years after Stranger Things was set.
It's leeching off far more than TXF, obviously.
It relies on its nostalgia in a lazy way and lets that be its shorthand for originality.
Regardless, the point of a cross-over is rarely to merge two already similar genres and styles. To the degree that X-Files crossovers work at all ("X-Cops" / "The Simpsons") is because the characters are hard pressed against a backdrop that is wholly different than TXF, not one that can be confused and derivative. The only thing you get out of that is a blurred brand.

It's an homage. It's supposed to make reference to those times and shows, right down to the girl's frilly Laura Ashley-esque dress.
I understand you think it's derivative, but to say it's leeching is more than a little harsh, IMO, especially since TXF didn't do its think until 1993, years after Stranger Things was set.

It's a good show in its own right. Why are you throwing shade on a show that helps get people into the genre?
Throwing shade? I think I'm allowed to like or dislike any number of shows based on any number of criteria without having my motives questioned.
The show deliberately attempts to tread on nostalgia and everything about Stranger Things, narratively, stylistically, aesthetically, production-value wise, is a throw-back to another show or movie that did it better the first time.

It's a good show in its own right. Why are you throwing shade on a show that helps get people into the genre?
Instant gratification takes too long.
-Carrie Fisher, RIP

Mighta been cool to see M&S show up on Stranger Things
Gag. Stranger Things is such a rip off of a million better movies and TV shows. Why should M&S appear in a show that is leeching off their own legacy?

yes indeed,JJ Abrams love the show,lost and fringe has a lot elements taken from x files

Mulder and Scully on "Fringe" would have been great!!

The creator of Babylon 5 really wanted to have Duchovny and Anderson show up as Psi Cops on an episode of his show but it never happened. I know it's not quite what you are asking though since they would not have been appearing as Mulder and Scully but it still would have been cool.