You mean like this?
THE WALKING DUCK DYNASTY!
Uncle Si, a glass of lemonade, tons of firepower…and Zombies!
Episode four : the gang, surrounded, trapped on the roof of the Duck Commander plant - Takes the decoys that were laying up there - Straps dynamite to the decoys and then throws them down into the crowd of Zombies who, mistaking the decoys for food, converge on the ducks and get blown up.
Uncle Si and Jase argue about which eyeball is the best for a zombie kill. Willie dismisses both with the comment, “just aim between the eyes.”
Wow, that’s awesome. Thanks.
X Files and Lost Girl
(Sample dialog:
Scully: She’s a succubus"
Mulder: Yeah, but she’s a GOOD succubus."
Scully (rolling eyes): I’ll BET she is.)
The Lone Gunman and The Big Bang Theory
Breaking both the rules set forth in the OP. I’d like to keep it “current shows on the same network”, please.
Doctor Who / Downton Abbey - The TARDIS materializes on the grounds of the Crawley family estate just in the nick of time to help the family and its plethora of servants fend off an invasion by Clockwork Robots being controlled by the Great Intelligence. Quintessential “base under siege” scenario - the Clockwork Robots have the house surrounded just as the family are hosting a masquerade ball. While the Doctor fights to fix a potentially devastating rift in the time / space continuum, Cora and Mary struggle to keep the party going without the guests getting wise to their mortal peril (“It will leave your party with the MOST DREADFUL reputation” harrumphs the Dowager Countess.) In the end, Edith decides she wants to see the universe and departs in the “Wellsian china box” that curious little man arrived in.
But OTHER THAN THAT … 
Doctor Who & Top Gear
We find out that the Stig is the Doctor.
What fun is that? 
I don’t watch either show, but I might watch that.
White Collar and Suits. Peter and Neal make an arrest on White Collar and who is that person’s lawyer, none other than Harvey Specter from Suits. Both on USA Network.
Exactly. I’m going to break the rules too.
The Love boat gets lost at sea only to wind up on Fantasy Island.
Walter from Fringe over does it with a new brand of psychotropic he concocted. After “testing” the drug, Walter finds himself right smack in the middle of Once Upon a Time In Wonderland.
Now that he’s retired from being in just one show, Richard Belzer (as his John Munch character) should be written into every current show that’s set in the present. He needs to make a cameo appearance in everything so all TV shows will be part of the same crossover universe.
Knowingly breaking the rules here but I would have loved to have seen Sam Beckett (Quantum Leap) leap into the body of a certain young female morgue attendant who has her own (somewhat limited) experiences with time travel (Tru Calling).
Instead of knowingly breaking the rules, why don’t y’all just start another thread?
Because the thread is more fun this way? 
Heh. That would get confusing.
OK, let’s try to follow the OP’s rules.
The Good Wife /* Elementary*: I have no idea what happens, but Will and Sherlock can be asses to each other.
Marvel’s Agents of S.H.I.E.L.D. / Back in the Game: One of the kids has powers or something, May’s crew investigate, the Cannon terrorizes these “Men in Black.”
Chicago Fire / Sean Saves the World: Hey, they’re both NBC and set in Chicago. Maybe Liz dates a firefighter. But which one? I’m going to say either Severide or Otis.
Bones / Sleepy Hollow: I’ve never seen more than pieces of Sleepy Hollow, but lots of supernatural weirdness gives Booth some really strange flashbacks.
Stick to the rules* in this thread, go here for expanded possibilities.
*the rules in this thread are for crossovers that actually could happen, same network, current run, etc…
*Agents of Shield/Castle *
Because Nate Fillion