Be certain that I mean a GUEST STAR, not a regular. I mean a one or two time appearance, not a reoccurring character. Also, I don't want to see a character that violates the nature of the show - for example, on Castle, a reality based no-nonsense police show, I wouldn't want Jeannie or Samantha Stevens; the existence of magic would destroy the concept of Castle and Beckett's investigations and showing a factual reason for every murder.

(Yes, I know a great story COULD make sense of such crossovers; but please omit them.)

However, don't feel restrained by different companies, productions, etc. I have one for my third example below.

What would I have liked? Well, there are the obvious... I'd have loved a crossover between the Addams Family and the Munsters.

And since I mentioned it, a teaming of Samantha and Jeannie could have been really fun.

When I was at that age, on both the Batman TV show and Filmation's' Batman cartoon, I really wanted Superman to appear... but that teaming had to wait for Super Friends.

That's a bare few that I have. Try to keep it to five or six if you would, and maybe an explanation if it doesn't seem patently obvious.

All of which means that Calista Flockhart's Ally could conceivably have shown up in that world, as could any of the other characters from her show. Peter MacNicol played Denny Crane's therapist in an episode and mentioned that his brother was a lawyer, perhaps hinting at a connection already.

So many possibilities with the tv westerns of the time.

Cain of KUNG FU wanders into GUNSMOKE'S Dodge city.

Earlier in GUNSMOKE'S run we could have Matt Dillon run into Paladin on HAVE GUN WILL TRAVEL.

That's..... actually pretty brilliant. It was established on RESCUE ME that the FDNY do not care much for the NYPD, so that would have made for an interesting dynamic  between Tommy and Andy. 

Doctor Who on one of the Star Trek shows is probably the obvious one. Not sure if that would violate the nature of the shows though. (Though the Doctor has been known to travel to other dimensions, and that was how the comic version of the cross over handled it.)

But failing that, Ash Williams (Bruce Campbell) on Supernatural. The hi-jinks and one liners that would ensue would really be something to behold.

Funny you should put it that way Brian. I read a while back that that was exactly the reason the writers put in that line. It was their tip of the hat to that possibility.

The early crossover/romance that took place between Ally McBeal and Bobby Donnell from THE PRACTICE I only discovered many years after the fact. I'd seen the Ally McBeal episode when it first aired in the UK, and always wondered why some of the plot didn't seem to be followed up on. It was only about 2 or 3 years ago that I finally saw The Practice side of it and finally everything made sense.

I suppose that was the disadvantage of watching Ally McBeal in the UK pre-internet. You would never have know that there even was another half to the story, as The Practice certainly didn't receive the same amount of attention in the UK as Ally McBeal did..

Perry Mason is hired by someone accused of murder. During the trial, he grills the officer who arrested his client: Some guy named Sgt. Joe Friday.

Denny Crane growls, "Did you say, 'Klingons?'"

At another point, while wading through a crowd of reporters, Denny tosses off the sound bite, "I have my own spaceship. I'm multi-talented."

The fourth-wall breaks and witty quips are a lot of fun on that show. I'm not surprised to learn there was some intent behind that mention of the therapist's brother. The writers seem too savvy for it to have just been a simple plot conceit or incidental line.

Vincent Price guest-starred on an episode of Columbo in a small role. It would have been nice to have seen him as the murderer. In fact, speculation on who would have made a good opponent for Columbo is a sub-genre of its own. Patrick MacNee was another actor who appeared off to the side, yet could have been compelling as the killer. MacNee made an excellent Satanic villain on the original Battlestar: Galactica. Diana Rigg would have been a charming and canny foe as well.

Denny Crane growls, "Did you say, 'Klingons?'"

At another point, while wading through a crowd of reporters, Denny tosses off the sound bite, "I have my own spaceship. I'm multi-talented."

+++++++

There was also a scene where Denny’s phone made the chirp-chirp-chirp TOS communicator sound when he answered it, as I recall.