Anyone in the old "Who's Who in DC" series. Really, pick a character who's still around, and Hilarity Ensues as you see how terribly different the DC Universe is some 20 years later.
You can do the same for The Official Handbook of the Marvel Universe. Take a look at the Book of the Dead and Inactive from the initial print and see who's still dead...
Amazing World of DC Comics: In July 1976, the magazine described the 1986 Great Disaster at DC Comics. 1986 was the turning point for The Dark Age of Comic Books. In 1986, concluded Crisis on Infinite Earths, which destroyed the DC Multiverse. And DC comics from October 1986 include Man of Steel #1, which began the revamping of Superman, and Batman #400, which was the last pre-revamp Batman.
The pivotal time will be October, 1986 ... and in that month, the future of the world will be decided. Either the path of the Great Disaster will be taken, and civilization will fall, or the path of sanity will prevail and the Legion of Super-Heroes will emerge triumphant a thousand years later.
In Birds of Prey, there's an issue where Joker becomes Iran - uh, Qurac's ambassador again and threatens to bomb New York City. He tells the people listening to look on the bright side, as "this will finally shut down Cats!" Obviously, at that time it was meant to be a dig at the surrealist play of the same name, but now most people think of the movie and maybe think the Joker had a point.
A 1997 story had the JLA take on a mad scientist who had created a "luck machine" that altered probability in his favor, letting him win the lottery, the Nobel Prize, and become President of the USA in short order. The JLA confront him in the Oval Office and destroy his device, but when reality reorders itself, the President who thanks the team isn't the right President either, and the team realizes that reality is still broken. What's so funny about this? The "wrong" President looks just like Sarah Palin.
Issue five featured Superman attending the funeral of Metamorpho (again...), and was one of the only ones there. When he questioned the priest about it, he was told that since Superman came back, everyone expected superheroes not to stay dead, so they'd lost interest in memorial services. The service took place in a park dedicated to fallen superheroes, and in an obvious bid to make the point that this wasn't always the case, that sometimes dead heroes stayed dead, the artist had four memorial statues in the scene: Ice, Oliver Queen (Green Arrow), Hal Jordan (Green Lantern) and Barry Allen (the Flash). The issue was also the debut and death of Tomorrow Woman. All of them, Rex and Tomorrow Woman included, have since come back from the dead—with Ollie being the only one whose return predated Rex's. Granted, the statues also included the members of the Justice Society who were killed by Extant during Zero Hour: Crisis in Time!, most of whom stayed dead — except for Hourman, who also cheated death.
The first arc would feature the Hyperclan, during their time posing as heroes, execute several people, including what was very clearly supposed to be Wolverine and Doctor Doom. During their brief time at Marvel, Grant Morrison wrote New X-Men and Fantastic Four 1234. Guess which X-Man was a key member of Morrison's X-Men team and who the Big Bad of 1234 was.
On the back cover of the Batman: A Death in the Family trade paperback, in which Jason Todd, the then-current Robin, was killed off, then-Batman editor Denny O'Neil jokingly said, "It would take a sleazy stunt to bring (Jason) back", though he did admit that he voted for Jason to live. In 2005, Jason was brought Back from the Dead.
In the afterword to The Return of Barry Allen, a storyline in which Barry Allen does not actually return (yet), Mark Waid hyperbolically describes being driven up a bell tower with a rifle out of sheer exasperation at people asking him to bring back Barry.
"What is it with you people?" I screamed. "Barry is Dead! Gone! Hearsed! Why can't you let him rest honorably, in peace?"
Comic Book Guy: I believe that's the sound the Green Lantern made when Sinestro threw him into a vat of acid. Eepaa!
Geoff Johns' Teen Titans run featured a fight between the Titans and the Justice League, with Green Lantern (John Stewart) sarcastically proclaiming "And people ask why there's no 'Teen Lantern.'" A Teen Lantern would eventually be introduced in Young Justice (2019).
The New Teen Titans at one point were forced to sell out for cash, with one of the licensed properties created from this being a 'Teeny Titans' Spinoff Babies show. The Titans recoil in horror upon seeing this slapstick comedy show with their faces on it. The concept would be revisited in serious fashion twice, first with Tiny Titans and then with the cartoon show Teen Titans Go!, the latter of which is still a Contested Sequel to this day. One wonders if the Titans would react to it in the same manner they did with Teeny Titans...
In the X Men New Teen Titans crossover, the Titans question why the X-Men allow a russian (Colossus) on their team, as it's the middle of the Cold War. Ironically, the Titans themselves would let a russian - Red Star - into their ranks eventually, also before the end of the Cold War.
Titans (1999): A H.I.V.E. mook considers trusting Nightwing, saying that even if he was using something like a Jedi mind trick, she'd rather be with the Jedi. Her companion replies, "not if you're a damned stormtrooper, Tonya!" Fifteen years later, The Force Awakens introduced a stormtrooper who performed a Mook–Face Turn as one of its main characters.
This panel◊ from Justice League America #33. At the time, it was meant as a dig at Barbara "Batgirl" Gordon's fate in The Killing Joke, but with the events of Countdown to Infinite Crisis (Max actually does shoot Blue Beetle in the head) it becomes hilariously prophetic.
In the 1970s Batman story "The Man Who Falls", which chronicles young Bruce Wayne's training to become Batman, Bruce meets with an FBI agent who says "we don't pull our piece much. We leave that to Efrem Zimbalist, Junior", in reference to the TV show F.B.I. which starred Efrem Zimbalist Jr.. Efrem Zimbalist Jr. would later be the voice actor for Alfred Pennyworth on Batman: The Animated Series.
Issue 3 of JLA: Year One (written in March 1998) has a moment that seems to be intended as an tragic moment, due to being set in the past; The Flash (Barry Allen) and Green Lantern (Hal Jordan) have a heart-to-heart conversation about the life expectancies of superheroes, which ends with Hal assuring Barry that "I predict we'll both live to a ripe old age". At that point in continuity, both Hal and Barry were dead. However, these days that moment has reversed into hilarious and heart-warming due to both having been brought back from the dead throughout the 2000's.
In the "World's Finest" where Robin (Tim Drake) and Superboy (Kon-El) first meet, Superboy wisecracks that even with the costume, he knew it wasn't one of the legendary Flying Graysons. Guess which Robin was made into a contemporary of Superboy in Young Justice cartoon? (although this could also be considered more Harsher in Hindsight)
Speaking of Tim and costumes, DC once released a book about Bruce Wayne's costume designs for him and his team (Knight Gallery). Tim's section 2 entries stating "yeah, cape-wings are stupid". Cue the New 52 reboot, and just guess what Tim is now wearing (and everyone is making fun of)...
In Messner-Loebs's run on The Flash, there were several references to T. O. Morrow being stricken with depression after seeing something unspecified and horrible in the future. 15-20 years of canon developments and potential things for a reader to dislike later...
In The Flash 80-Page Giant #1, Mark Millar, appearing As Himself, notes that the in-universe version of DC Comics resolved all the continuity issues created by heroes going public with their identities with a crossover called "The Identity Crisis", apparently their equivalent of Crisis on Infinite Earths. Our DC would later use that name for a crossover themselves, though it actually didn't change continuity all that much.
Back when Two-Face was still known as Harvey Kent, the editors decided to give him a happy ending, and his face was restored by a doctor. And the doctor's name? Ekhart. In The Dark Knight, Aaron Eckhart would play Harvey Dent.
Some older comics contain fan letters from one Geoffrey Johns, who had suggestions like "What if Superboy's DNA was half Lex Luthor?" and "Could you do another story with [dead at the time] Professor Zoom?" He got responses like "Sorry, in the time since you sent your letter the other half's been revealed as Paul Westfield" and "Sorry, Professor Zoom's dead and we're cutting back on time travel stories." Then he grew up and made his dreams come true.
The second issue of Grant Morrison's Animal Man run has a scene where Buddy runs into a teenager in LA named "Jaime", who collects autographs from famous superheroes. When he opens up his autograph book to try to get Buddy to sign it, the first visible page reads, "To my good friend Jaime - The Blue Beetle". That issue came out about 18 years before a teenager named Jaime Reyes officially became the third Blue Beetle in Infinite Crisis.
The Marvel Comics character Black Cat is often called a rip-off of Catwoman. At one point, Anne Hathaway was set to play a heavily modified version of Black Cat in the fourthSpider-Man movie, but the film was never made. The big superhero movie role Hathaway did end up playing years later? Catwoman, a.k.a. Selina Kyle, in The Dark Knight Rises.
In the Deathstroke the Terminator tie-in with the Titans $ellout $pecial, a toy company is producing a Deathstroke action figure set, including "Wintergreen: Friend from Down Under!" When one of the execs points out Wintergreen's British, they're told that Australia is cooler. In Arrow, not only is Wintergreen an Australian, so is Slade himself!
In an old issue of Justice League International, Booster Gold and Blue Beetle tried to recruit new members for the JLI, but met with failure at every turn. Eventually, the two suggested that they get Congorilla to join the League in a very derisive "We're really scraping the bottom of the barrel" way. Decades later, Congorilla did actually join the team in the Justice League: Cry for Justice series.
In the MAD parody of the 1966 Batman (1966) TV show, the villain who's been trying to kill Batman turns out to be Robin in disguise. Fast-forward 35 years to Frank Miller's The Dark Knight Strikes Again, where the maniac posing as the new The Joker, who's been taking out heroes left and right turns out to be...Dick Grayson.
During the JLA/Avengers crossover, Captain America sees the Jason Todd costume hanging in the Batcave and is reminded of the death of Bucky. Both Bucky and Jason Todd would be resurrected in the exact same year.
In the Batman arc "Shaman", Batman ambushes a group of thugs in a drug deal and interrogates the last one, saying "So if you don't tell me every little thing I might want to know, you'll be going to jail in a baggie. Believe it." "Believe it" would go on to become the catchphrase of the title character of Naruto, establishing an amusing connection.
The cover of Action Comics #371 (here) comic, depicting Superman's face turning various colours from different emotions. Including "yellow from fear".
Action Comics (New 52) had a one-shot issue about an Alternate Universe version of Superman who literally began life as a comic character. In the story, it's stated that the corporation that owned Superman made him Darker and Edgier to appeal to a modern audience, thus missing the entire point of the character. Then Man of Steel came out the following year, with one of the major complaints being that WB had tried too hard to make Superman Darker and Edgier, to the point that he killed General Zod.
Then-current Adventures of Superman writer Jerry Ordway would often say "Let's just kill 'im!" whenever the Superman writing team are stuck on ideas. When their proposed "Superman marries Lois Lane" story arc was temporarily shelved to avoid conflict with the Lois & Clark TV series (which also planned to marry Superman and Lois in a future episode), Ordway, at the next meeting, said, "Let's just kill 'im!" The end result was The Death of Superman.
In The Living Legends of Superman, the Man of Steel travels to the future and meets a friendly Superman fanboy called Riley Bendix. Thirty-eight years later, a Superman writer would create a villain called Bendix which would pester Superman's son.
"Superman vs. Muhammad Ali": This 1978 comic features an alien race called the Scrubbs. Many decades later, "Scrubb" would become an online insult.
In Action Comics #270 (1960), Superman dreams he has travelled to the future, and Linda Lee works as a reporter in the Daily Planet. Linda never was a reporter in the comics, but in 2016 she became one in her live-action show.
Linda Danvers worked for San Francisco news station KSF-TV in the 1971-1972 period. Some years later a real life San Francisco TV station with call letters similar to the one in the Supergirl comics was started. The station is KTSF-TV, channel 26. It is an independent station broadcasting mostly in Chinese, serving the Chinese community there.
In Supergirl (1982) 2 #17, Linda thinks about adding glasses to her disguise but discards the idea. Flash-forward several years and her disguises usually include glasses.
In an issue of Justice League Task Force, Maxima expresses confusion at the idea of same-sex relationships. Supergirl (2011) story arc "Crucible" reimagined Maxima as a lesbian.
In "The Super-Steed of Steel", villain Vostar assumes Comet the Super-Horse to be a Kryptonian animal because it is a flying creature who hangs out with the Super-Family (unaware that Comes is only an Earth creature with super-powers). Twenty years later, comic fans assumed Comet to be Kryptonian for the same reasons, and used him and Streaky (who was not Kryptonian either and had not even showed up in one decade) as an excuse to complain about the supposedly excessive number of Kryptonian survivors "flooding" the Superman books.
The Supergirl from Krypton (2004): During Supergirl's debut, Superman, annoyed by Batman and Wonder Woman's hesitance to go after Kara, unintentionally offended them by bringing up Jason Todd and Donna Troy, both of whom were dead at the time. A year later, as part of the lead-up to Infinite Crisis, Jason and Donna came back.
In 1963 story "Supergirl's Big Brother", when a man knocks on the Danvers door and is greeted by Linda Danvers, he claims to know for a fact the Danvers don't have a daughter because he is their son Jan. Linda retorts she is their adoptive daughter, and their parents never mentioned one older son. Fifty-two years later, the Supergirl (2015) show introduced a gender-flipped version of Jan Danvers.
Supergirl Special: One of the criticisms spouted by proponents of the Post-Crisis reboot is Pre-Crisis Krypton allegedly looking too much like a 20th century America where everybody wears gaudy costumes and lives in metal spires. Nonetheless, Post-Crisis stories like this one-shot ended featuring Kryptonians having English-sounding names, wearing Earth-looking clothes and practicing Earth sports without the lore and cultural nuances which made Pre-Crisis Krypton feel truly alien.
A comic-book reviewer joked about rainbow Kryptonite that turns Superman gay. Later on, Many Happy Returns would introduce pink Kryptonite, which reverses nearby Kryptonians' sexual orientations.
Batwoman was introduced as a love interest of Batman as a way of combating accusations of him being gay. The character would eventually be reinvented as a lesbian, with the express purpose of creating an Affirmative-Action Legacy.
The first arc of Batman: Shadow of the Bat, "The Last Arkham", features the debut of Arkham Asylum director Jeremiah Arkham and the title presumably refers to him. Come 2018 and we learn he isn't the last Arkham as the DC Rebirth version of the Arkham Knight is revealed to be Jeremiah's daughter, Astrid.
James Rolfe & Mike Matei themselves would appear in an issue of the TMNT/Batman crossover comic book series on a panel in which Michaelangelo runs into a pizza shop screaming, "Everybody run! There's a crazy guy in a bat suit trying to kill us!". We've come full circle.
Lyta Milton has purple hair and is the daughter of an evil Olympian god (Ares) and an evil sorceress (Circe). Some who are unfamiliar with Wonder Woman's more obscure supporting characters would be forgiven for seeing her as a ripoff of Mal from Descendants even though Lyta debuted two decades earlier.
In Power Girl issue #7, PG briefly fights the Wonder Woman villain, the Blue Snowman, who is a girl, and the captions lampshade this by describing her as a "Wonder Woman villain with gender issues". At the time, this was just a joke, and Blue Snowman was just a case of Sweet Polly Oliver - but then they gave the Blue Snowman some actual gender issues in Rebirth, with the reveal that that incarnation of the character was assigned female at birth.
the Joker had a point.
◊ from Justice League America #33. At the time, it was meant as a dig at Barbara "Batgirl" Gordon's fate in The Killing Joke, but with the events of Countdown to Infinite Crisis (Max actually does shoot Blue Beetle in the head) it becomes hilariously prophetic.
