Here are my headcanons for non-character related things in Cats. Please drop a note in the reviews or PM me for more ideas of things I should include in this section.

Special thanks to CoseyRosie for helping me with the section on Skimbleshanks' train!

The Mating Dance

I know Plato does it with Victoria in the movie, but in my world/headcanon/AU (somebody please let me know which word is correct to use there), Pouncival does it with Victoria instead. Everyone lays down to rest and she doesn't, and because he thinks she's hot, he asks her to dance. She is hesitant at first; generally she only dances with Misto because she knows he won't try anything weird. But Pouncival encourages her on the basis of it being "the adult thing to do." Dancing with him, she realizes that sensual interaction can be nice and begins to open up to her own blooming sexuality.

According to the Cats Musical Fandom Wiki, the two cats who do the mating dance (officially called the pas de deux) differ in each production. So that's where this idea came from*

Growltiger

My Growltiger headcanon revolves around Broadway-style productions. The head Siamese cat is Genghis (rather than Gilbert) and the duet that Growltiger and Griddlebone sing is the Italian Aria, In Quella Tepida Notte, rather than Billy McCaw.*

The story of Growltiger is a feline tall tale. Growltiger, Griddlebone, and Genghis were all real, but the story is highly exaggerated, to the point where nobody knows for sure which aspects really happened and which didn't. It has been turned into an opera in the Jellicle world, which Gus starred in. The Growltiger song in the show is a snippet from that opera.

Skimbleshanks' Train

The Northern Mail, Midnight Mail, Night Mail, and Sleeping Car Express are all the same train, which is officially named The Northern Flyer. It's a passenger train with a mail car (or cars). It goes from London, England, to Glasgow, Scotland, and back, leaving around midnight so the passengers can be at their destination by morning. Skimble is always on this train as opposed to going between different ones.

The Battle of the Pekes and the Pollicles and the Rumpus Cat

This one is an urban legend/tall tale as well. It is true that there are two gangs of dogs, the Pekes and the Policies, who hate each other with a burning passion. The Rumpus Cat bit may or may not have really happened. The Rumpus Cat is a superhero character to the Jellicle Cats; the only difference is, he might have been real. Most of the cats believe he is or was real, but no one has concrete proof one way or another. The song in the show is a play organized by Munkustrap performed for Old Deuteronomy; every year, upon his arrival, the tribe puts on a play for him. Admetus portrays the Rumpus Cat in the play.

Cat Curses and Sayings (NOTE: I did not invent most of these.)

The Jellicle Cats use some of the same curse words and sayings as Humans do, but they have some of their own.

Everlasting! or Everlasting Cat! - Equivalent to Human "My God!" Most cats use it freely but it can be considered taking Everlasting's name in vain depending on what cat you're talking to.

By Heaviside! or By Everlasting! - Equivalent to Human "By Jove!"

Everlasting Knows or Heaviside Knows - Equivalent to "God knows," "Goodness knows," or "Heaven knows."

Bast! or Bastet! - Equivalent to Human "Dang it" and similar phrases. Like "Everlasting Cat" but classier and generally used more lightly.

Well I'll be a Pollicle's Uncle - Equivalent to Human "Well I'll be a monkey's uncle."

Bast-Damned, Bast-Darned or Bast-Danged - Equivalent to human "gosh-darned," etc. Used instead of Everlasting-damned or Everlasting-danged because that's considered crude.

Cat-Awful or Everlasting-Awful - Equivalent to Human "God-Awful"

Cat/Everlasting/Heaviside Bless It -

Euphemism for Cat-dangit, etc.

Cat/Everlasting/Heaviside Bless (You) - Use 1. When someone sneezes. Use 2. Thanking someone ("You've been such a big help. Cat bless you.) Use 3. Revering the deceased ("Her mother is dead, Cat bless her.") Use 4. As in Human "bless your (pea-picking) heart", which can be both sarcastic and genuine.

Use 3 may also be worded as "Cat rest." (Her mother is dead, Cat rest her.")

Honest to Cat - Equivalent to Human "honest to God."