If the Halloween special episode was also the Big Reveal, I'd make it something like this:
Its Halloween. Everyone has to dress up in a costume. Marinette considered being a witch, but she drops the idea when she sees Chat Noir running over the rooftops of Paris to save someone who fell off their balcony or something the day before.
+Flashback to yesterday+
Marinette quickly makes a note of the costume she's going to wear and transforms into Ladybug. By the time she gets to the place, Chat Noir already caught Alya's little sisters, one in each arm.
Ella: Chat Noir!!!😄
Chat Noir: Why, hello, little kittens! Where did you fall from?
Ella and Etta: *cute giggles* Up there! *points to their balcony*
Chat Noir: Hang on tight!
Both the girls hang on to his neck from behind to free his arms. He extends his staff vertically upwards and lands on their balcony. He puts them inside and they both give him a hug.
Chat Noir: Now be more careful, okay, kittens?
Ella and Etta: Thank you Chat Noir! We will!
Chat Noir: *stands up, smiles and slides down his staff* Huh? Ladybug! What took you so long? *flirty and playful*
Ladybug: Your kittens seemed really thankful for your help. *giggles*
Chat Noir: Oh c'mon, they're adorable!
Ladybug: Just like their daddy cat.
Chat Noir: *blushes hard*
Ladybug: Well then, I guess there's only one thing left to do before we go.
Chat Noir: Hm? What's that?
Ladybug: I need to ask you something. Tomorrow is Halloween. So, how do you think we should dress up in a costume?
Chat Noir: We could put something over our suits.
Ladybug: That could work. I guess I'll see you tomorrow then! Bye!
Chat Noir: Sure. Bye!
Ladybug throws her yoyo and zips away. Chat Noir just decides to walk home and sneak into his room. Once he's in, Plagg decides to detransform and have his cheese break (again).
Plagg: Kittens, huh? That's cute.
Adrien: You have to admit, Alya's sisters are cute. Almost all little kids are cute.
Plagg: They sure are. *bites cheese* So, what are you gonna be for Halloween?