There was a time-travel arc. Why not an arc on Parallel Universes?
This is a great idea!
Most of the ideas I originally came up with have been done already in a GWC arc somewhere.
New Ideas:
*The Golden Compass
*12 Monkeys?
*Alice in Wonderland
*Stardust
*Neverending Story
Sliders seems obvious, as does Fringe. I’m also a big fan of Frederik Pohl’s The Coming Of The Quantum Cats.
And of course you have to include a discussion of how a goatee means you’re from an evil parallel universe.
I love the idea!
Watchmen?
lb
This has already been done; oh wait… That was another GWC in another parallel universe, sorry… :o
giggle 
Little more alternate history, but there are a bunch of great books, ie bitter seeds.
“Ring Around the Sun” by Clifford D. Simak comes to mind. Also, the Darktower series by Stephen King.
Both the World War and Southern Victory series by Harry Turtledove are choc full o’ awesome.