The Arrowverse has grown far larger than perhaps anyone would have expected. There are countless easter eggs to look out for throughout their various shows, but not all of them actually get paid off. In regards to the larger Multiverse, many of these major items, settings, and characters have come into play; in the Arrowverse they have yet to make a full appearance though.
It's not always clear as to why an easter egg is forgotten about. It could be that plans have changed, or it was only ever meant to be a fun nod for fans of the comics books. Regardless, there's clearly a lot more canon mythology to explore in the CW's version of the DC Universe.
The Green Lantern Ring
The finale of Arrow allowed fans to live out a much anticipated and theorized moment. Crashing down from the sky, a green object appeared in front of John Diggle, cementing his future as a superhero. The assumption was of course that Diggle would become the new Green Lantern.
However, there's been no payoff from this moment since. Perhaps it was never a ring at all but another green object; maybe even Kryptonite. With no clear answers, this remains an easter egg with little substance, teasing fans that their hopes for Diggle had been fulfilled without committing to them.
Bane's Mask
When fans met Nora Fries and her Mister Freeze weaponry, the assumption was that her husband would appear down the line, possibly in Batwoman. While he still hasn't made an appearance, this impressive technology wasn't the only easter egg that's been put on ice.
In the background of these Elseworlds scenes is Bane's mask from The Dark Knight Rises. It's never really explained and the formidable Batman villain hasn't made his debut in any Arrowverse show. Maybe it's teasing an even larger Multiversal crisis that hasn't happened yet.
The Hall Of Justice
The finale of Crisis On Infinite Earths contained a number of easter eggs including the use of a STAR Labs facility to house a new superhero team. Of course, they were never given the official title of the Justice League, but Winn Schott has noted that the building was referred to as the Hall of Justice in the future.
It was a great nod to the famous DC team to have this facility present in the Arrowverse but it has since been left to gather cobwebs. With no major threat inbound, perhaps the Justice League will never actually team up again in the traditional sense fans are used to.
Blüdhaven
Blüdhaven somehow became a major city in Arrow with Roy Harper even moving there for an extended period. Plenty of characters made their way to the famous DC City and this should have all been in aid of the eventual live-action appearance of Nightwing.
This Bat-Family member has become the iconic hero of Blüdhaven and it was as if all signs were pointing to the vigilante's appearance. Instead, the city was used like any other location, with its name drawing less and less meaning each time it was showcased.
Coast City
Speaking of notable locations, Coast City has also been mentioned again and again. There have been billboards advertising the vacation destination and a former partner of Oliver Queen, McKenna Hall, actually moved there after sustaining injuries in the field.
This is the home of Hal Jordon, with the Coast City easter egg indicating that the Green Lantern might enter the Arrowverse. This has still never really happened, although HBO Max is currently working on a series that may be linked to the larger DC Multiverse.
Themyscira
The Legends have traveled to many incredible destinations but one that caught fans off guard was Themyscira. In the comics, the famous Greek location is home to the Amazonians and most notably Wonder Woman. Diana Prince hasn't made an appearance in the Arrowverse yet though.
The Legends did drop off Helen of Troy in the location though, which suggests that there could be plans to revisit in the future. Regardless, this is a great easter egg for fans of the DC mythology, and it's a shame that thus far it hasn't been mentioned since.
Gleek
The Wonder Twins are also synonymous with their strange pet monkey Gleek. The primate is the kind of character that would never really appear in live-action, but there's a major easter egg for him at the end of Crisis On Infinite Earths; one that's been forgotten about though.
At the back of the Hall of Justice is a cage that has the word Gleek on it. Monkey-like sounds can also be heard, suggesting that the animal has gotten loose somewhere in the facility and is ready to cause chaos in the next phase of the DC Universe.
Harley Quinn
The original Suicide Squad created in the Arrowverse has been quickly torn apart due to its big-screen counterpart getting preferential treatment. Arrow must have had big plans for the team though as they teased some other players who could join the ranks.
The major moment was seeing the back of Harley Quinn's head in a cell, with the assumption being that she would soon be unleashed. Quinn has never been seen again, but the Joker has been confirmed to have been killed by Batman in the universe. For now, Margot Robbie still holds the crown as the only version of the character.
General Zod
Micahel Shannon might have been a superb General Zod previously, but it looked like the Arrowverse was going to go in a different direction. In Clark Kent's hallucinations of his past, fans got to see Zod up close for the very first time.
Even though Superman And Lois is now ongoing in this new post-crisis Multiverse, the original Zod hasn't actually been brought back. He was written out of the storylines after being abandoned in the Negative Zone but it would have been fun for the initial easter egg to pay off.
Atom
The first Atom in the Arrowverse, Ray Palmer, has been written out of the Legends Of Tomorrow storylines after getting married to Nora Darhk. But in the Crisis On Infinite Earths, a brand new Atom was being set up to take his place in the field.
Ryan Choi was introduced to audiences. While it seemed that he would be picking up the mantle down the line this was clearly more of an easter egg for comic book fans. The character hasn't been utilized since and there doesn't appear to be any plans in place.