Connections to previous Batman films

Actors who played Penny Fleck and Thomas Wayne previously appeared in Catwoman and Dark Knight Rises

Joaquin was considered to play Batman in Darren Aronofskys Batman Year One, and De Niro as Two Face in Batman Forever

Alec Baldwin was initially cast as Thomas Wayne but didn't due to scheduling conflicts. Baldwin was considered for Batman in 1989 and Forever in 1995 (and played the Batman like Shadow in 1994, made due to the success of Bat89).

Arthur Fleck = A.Fleck = Affleck/Ben Affleck?

Arthur in the police car watching the events in Gotham, leaning against the window in similar pose as Heath Ledger in The Dark Knight

"Put on a happy face!" - a quote from Batman 89

Bruce sliding down on the poll - Batman 1966