I think a Agents Of S.H.I.E.L.D./Once Upon A Time crossover would work-A group that investigates strange goings on in a town full of strange goings on. Over on CBS I’d like to see an Elementary/Mentalist meeting of the minds.
The Big Bang Theory and Two Broke Girls. 
Especially before Howard got married. Can you imagine.
Don Draper battling (or trying to sell stuff to) The Walking Dead?
Lena Dunham and her friends start sleeping with vampires on True Blood?
Elementary & Person of Interest. Thread over. 
Elementary/Mentalist/Person Of Interest totally hetero three-way.
NCIS Los Angeles needs help solving a case involving advanced physics. They contact *Big Bang Theory *, especially Sheldon Cooper. He gives them advice and his eidetic memory proves key to them solving the case. At the close, G walks over to his supervisor’s desk and says “Hetty” (knock knock knock), “Hetty” (knock knock knock), “Hetty” (knock knock knock)…
Can’t think of any offhand but Felicia Day really needs to guest star on The Big Bang Theory.
We’re running out of time for Sara Michelle Gellar and Alyson Hannigan to meet up on a Crazy Ones/How I Met Your Mother crossover, with bonus points if Robin Williams can hear the narrator.
Doctor Who/Star Trek
Way to violate both rules.
Maybe he’s writing from a parallel universe.
I’ve said this before (in a thread about crossovers that can only end in violence) but I’d like a NCIS (orginal flavor)/ Mentalist crossover.
Once Upon a Time / Lost
(although I get the feeling Once Upon a Time is headed in that direction anyway.)
eta: Sorry … I know I violated #2, but it’s still an interesting thought.
The ISIS field team is sent to rescue a middle-aged comedian who’s being held hostage at a farm outside Beijing…
Supernatural/Sleepy Hollow. I’d love to see how the Winchesters would handle the Headless Horseman/Death. “Wait, you think you’re Death? I knew Death. Death was a friend of mine. Horseman, you’re no Death.”
Until that happens, you have to settle for print
I would had like to have seen the cancelled Law & Order crossover that involved the then three franchises (Law & Order, L&O:Special Victims Unit and L&O: Criminal Intent) about terrorist acts in New York. Supposedly planned for the 2001-2002 season, then an actual one happened.
Current one, “Psych” and "The Mentalist, " maybe throw in “Elementary” in there.
Unfortunately, “Psych” is from another network.
Sherlock/Doctor Who.
Well, Richard Belzer as “Det. John Munch” crossovered several networks.
However, I remembered another reason why it can’t happen. In “Psych,” “The Mentalist” is a TV show.