what shows do you wish had done a crossover?

Bewitched and I Dream of Jeannie
Frasier and The Nanny- While the style of these 90s comedies was totally different, they both had major characters named Niles.
The Mary Tyler Moore Show and WKRP in Cincinnati
Law and Order and Law and Order: Criminal Intent- While CI is techinically a spin-off, it is strange Goren and Eames were not introduced on the parent show first. Plus Briscoe and McCoy could have crossed over to Criminal Intent.
Blue Bloods and NCIS
The Waltons and Touched by an Angel
Dragnet and Highway Patrol- Dan Mathews and Joe Friday teaming up
Burns and Allen and Green Acres- Gracie Allen and Lisa Douglas would make quite the pairing.
Wings and Night Court- Bull and Lowell would be good friends.
Young Sheldon and The Middle
Family Affair and The Patty Duke Show
Bachelor Father and The Patty Duke Show
WKRP in Cincinnati and Head of the Class- with Howard Hesseman playing both his roles
Welcome Back, Kotter and Head of the Class
Welcome Back, Kotter and Green Acres
Family Ties and Growing Pains
Family Ties and Head of the Class
Coach and Wings
Mary Tyler Moore and Too Close for Comfort- with Ted Knight playing both his roles
Home Improvement and Coach
Family Ties and Home Improvement
My Two Dads and Mad About You- with Paul Reiser playing both his roles
I Love Lucy and The Dick Van Dyke Show
The X-Files and Star Trek: Deep Space Nine
The X-Files and Star Trek: the Next Generation
Corner Gas and The Middle
My Two Dads and 80s Punky Brewster
Touched by an Angel and 7th Heaven
Monk and Psych
The Middle and Reba
Gilmore Girls and Hope and Faith
Bachelor Father and The Patty Duke Show
Gomer Pyle, U.S.M.C. and C.P.O. Sharkey
Family Affair and Leave it to Beaver
The Practice and Law and Order
I Love Lucy and The Andy Griffith Show
The Andy Griffith Show and The Dick Van Dyke Show

Sanford And Son meets What's Happening

I'd like to dee THE DICK VAN DYKE SHOW and THE ANDY GRIFFITH SHOW. Maybe it could be; the Petries are driving to or from somewhere and they're passing through Mayberry when they get pulled over for speeding and Rob gets put in jail, while Richie and Laura stay at the Taylors.
Or THE ODD COUPLE and BARNEY MILLER. Maybe-Oscar gets arrested for something and Felix comes down to bail him out.

The Jeffersons and Diffrn’t Strokes

Switched at Birth and Make It or Break It
The Wonder Years and The Kids Are Alright

Don't remember if these two shows were on the same time but how about Murder She Wrote Jessica Fletcher meets Bob Newhart at the inn in Vermont.

Buffy The Vampire Slayer - Dark Shadows

Major Crimes and Blue Bloods