Headcannons!

Chapter 2: Siblings!

HARLEY & HARVEY: They have matching names. Okay, so what if they are siblings (or half-siblings), and Harley changed her surname when she became a psychologist at Arkham, because her patients wouldn't listen to her if they knew her brother is the one who put them away. The idea started off just as a joke because of their names, but now I actually really like it. Also, they both have high-up jobs working with criminals, and both have mental issues. 

BRUCE & SELINA: Okay, okay. I know! But hear me out, what if their flirty thing is replaced with sibling banter? They would know how to push ALL of each other's buttons, and it would give a similar reveal: learning that his sister is Catwoman, a supervillain? Anyways, I feel like it actually makes sense. Their parents die, he leaves Gotham, she stays. Maybe she gets taken in by Sewer King? She becomes a thief, blaming the corrupt rich of Gotham for her parent's death. You honestly wouldn't have to change all that much. They even have matching themes! Tight, black & yellow, Cat/Bat suit. I think it would work well for those who ship the characters with other people (like her with Penguin, or him with Joker), and still want to keep their dynamic.

CLOCK KING & MAD HATTER: This is actually cannon to one old comic (Batman 66' I believe). they both have similar mental issues, they are both English, their themes also kinda match. I can see Jervis (MH) calling Temple (CK) 'White Rabbit', for his whole time thing.

They have different surnames because Temple got married and changed his name.

PENGUIN & NORA FRIES:
okay, so, I used to headcanon that Mr Freeze and Penguin were siblings (because of the shared cold thing), but then I had another idea... What if Nora and Penguin are? It would make sense why he funds Freeze's work, he's trying to save his sister. Also, Nora's cryotank could be kept in the Iceberg Lounge, in it's cold rooms. Victor And Nora could have chosen their last name (Fries), because of a shared love of the cold between her and her brother (Oswald). Also, (according to some things) they were born around the same time. I think it would just be really interesting if they were siblings.