The former residents of the Queen Mansion.
When presumed dead billionaire playboy Oliver Queen, heir to the Queen fortune, returns after five years away, something felt familiar about the family's mansion. While Arrow certainly shares some cool stylistic details with Christopher Nolan's Dark Knight trilogy, Hatley Castle in British Columbia was never Wayne Manor. No, the real life location once served as the cinematic home for Charles Xavier's School for Gifted Youngsters in X-Men. And it doubled as the Luthor estate in Smallville. And it was last used in the upcoming sequel Deadpool 2.
Liam Neeson offered to appear on Arrow.
When producers and fans started talking about the forthcoming appearance of Ra's Al Ghul on the show, Liam Neeson told an interviewer he'd be interested in reprising the character, which he first played in Batman Begins. Presumably, nobody took Neeson up on his interest in order to maintain distance from Nolan's trilogy, itself separate from the DCEU, which is separate from the Arrowverse. (Neeson did turn up on TV reprising another iconc role, however, making a handful of appearances in Star Wars: The Clone Wars series.) As Arrow fans know, Ra's Al Ghul became one of the most significant and best mentors, frenemy, and big bad for Arrow. He's someone with important ties to characters like White Canary and Malcolm Merlyn, the first season's Dark Archer who briefly held the mantle of Ra's Al Ghul. The Nolan trilogy renamed his group The League of Shadows. The Arrowverse adopted the name more familiar with comic book readers: The League of Assassins.
Diggle's real life inspiration.
The Dark Knight trilogy owes much to Frank Miller's work in the comic books and in similar fashion, the TV Arrow owes a great debt to the fantastic Green Arrow: Year One. The influential mini-series was written by Andy Diggle, a British comic book writer responsible for kickass stories starring Judge Dredd, Hellblazer, The Losers, Swamp Thing, Batman, Daredevil, and The Punisher. John Diggle was named in tribute to the real-life Diggle. Eventually on Arrow, fans were introduced to Dig's wayward brother, once thought murdered by Deadshot: Andrew "Andy" Diggle.
From Green Arrow to Arrow to Green Arrow to Arrow...
Nick Fury didn't always look like Samuel L. Jackson in the comic books, but he does now. Like many great comic book based properties before it, the superb writing and casting on Arrow has in turn influenced the modern comic book iterations. Oliver's best friend, bodyguard, Team Arrow teammate Spartan, and sometime Green Arrow himself, John Diggle, was created for the TV show. He became so popular that when DC rebooted their comic continuity with the New 52, Dig was now in their pages, too.
Harley Quinn exists in the ArrowVerse.
The Joker's favorite little monster originated on Batman: The Animated Series and like Diggle, became so popular she eventually joined the comic books. Diggle was actually just a few feet away from Harley Quinn at least once, in the Season 2 episode that introduced the Arrowverse's "Suicide Squad." Despite her pointing out she's a trained therapist, she's denied the chance to come on the mission. Quinn was played by actress Cassidy Alexa who can be seen from behind in a deleted scene.