I have thought for many years tv companies should have done a crossover of series.
For example Only Fools and Horses with Cole as Delboy, Waterman as Rodney, possibly Warren Mitchell as Albert, Rula lenska as Raquel, Ian Mcshane and Ray Winstone as the Driscoll brothers and Pat Malahide as Slater.....
....With Minder, David Jason as Arthur, Nick Lyndhurst as Terry, Ken Mcdonald as Dave and Jim Broadbent as Chisholm.
There are quite a few instances of this in 'Endeavour', well they are not that obvious but they amuse me. I actually am a big fan of 'Endeavour' but I have still not watched that many episodes. However, I have tried to catch up with series 5 and 6 and it is there I have noticed a few crossovers, or I suppose references would be a more accurate term. The first and most obvious one was when Morse gets to visit Crossroads Motel. He has to go to investigate some goings on at a motel near Birmingham. On the way he passes a signpost for King's Oak and then turns in to Crossroads. The sign outside is an accurate pastiche of the sign that used to appear in the credits of the soap and when he goes in Jill Harvey (or a young actress playing her) is behind the counter and shows him the crime scene.
Anyone not old enough to remember 'Crossroads' will not pick up the reference and they don't linger on the details or add background music to underscore where he has gone. It is played very seriously and is all the funnier for that.
US Cop Shows "Homicide" and "Law and Order" had a few, including at least one storyline that started in one show and concluded in the other. "Homicide" character John Munch played by Richard Belzer later became a regular in one of the other "Law and Order" series, and also appeared in the "X-Files" (which itself teamed up with documentary show "Cops" on one occaision).
Maybe not an actual TV show... a cartoon seen on TV. The Simpsons entering the Family Guy world. The two laziest husbands, the two more over-worked wives, with son, daughter and baby. Titles The Simspons Guys. It was a Family Guy episode.