My two favorites are "two characters have to pretend to be a couple on a case" and "surprise we have a dog/cat now".
You know, the first one is actually my favourite (and applicable to most of my OTPs), but the second is the prompt that wanted to be written so please enjoy this Hartwin ficlet.
Harry stared at the wriggling thing in Eggsy’s arms. “That is a cat.”
“Good to know losing an eye didn’t take away your keen observation skills, luv,” Eggsy said, not looking up from where he was trying to towel the fidgety creature dry. “And before you ask, there ain’t a collar or evidence of a chip, JB isn’t trying to eat ‘em and it’s pissing down out there so I ain’t letting the cat back outside.” He looked up at his husband. “Any other questions?”
Smiling at Eggsy’s infinitely-tender heart, something not even his work as a Kingsman has yet tarnished, Harry reached out to stroke the tabby’s head. “What shall we call it?”
“Her,” Eggsy said, smiling the way that lit his whole being up. “We’re keeping her?”
“Once we’ve done due diligence trying to find an owner,” Harry said. “If one does not turn up how do you feel about Morgana for a name?”
Eggsy pursed his lips. “She ain’t a Kingsman, she ain’t getting a Kingsman name.” She regarded the now still cat. “She can be Chloe,” he grinned, “to go with Jack Bauer.”
Harry tilted his head. “JB stands for Jack Bauer?”
Blinking, Eggsy asked, “Have I never told you that? We’ve been together for over five years, luv.”
“I thought it stood for James Bond,” Harry crossed his arms.
Eggsy laughed so hard, Chloe squirmed out of his arms and headed out of the loo. Harry joined in and Chloe fled the sound of their loud laughter to take refuge with the friendly pug she’d met earlier.