Side note on “crossovers we wish could have happened”:
I would have loved to see an X-Files/Early Edition crossover.
Still wish Mythbusters meets Rocket City Rednecks could happen.
There was an episode of Roseanne where Roseanne Connor met Edina Monsoon and Patsy Stone at a party. That’s not only a cross-network (ABC & the BBC) crossover, but an international one too.
…the duo of Eli Levinson and Denise Iannello (played by Alan Rosenberg and Debi Mazar) moved from Civil Wars (ABC) to LA Law (NBC).
Dharma and Greg was an ABC show produced by Chuck Lorre. The two characters later made a cameo appearance on Two and a Half Man, a Lorre show on CBS.
When the Mission: Impossible series got revived in the '80s, it had jumped networks from CBS to ABC, giving everyone from Peter Graves to Greg Morris to Lynda Day George a chance to reprise their roles; for bonus points, this also ropes in the arguable Barney Collier appearance on The Jeffersons.
No, the characters didn’t cross. The actors did. (There was a joking reference to the actor having been on the soap, but it wasn’t a character move.)
Also, an actor reprising one of their famous roles years after that show has gone off the air isn’t a really a crossover. A crossover is when two currently airing shows collide.
David E.Kelley and Chris Carter supposedly wanted to do a Picket Fences and X-Files crossover, but never got approval.