2012-2013
Arrow Season One: Oliver fights Merlyn. Flashbacks feature Oliver v. China White.
2013-2013
Arrow Season Two: Oliver vs. Deathstroke. Barry Allen is introduced in one episode. Sara Lance would probably die before the end of the season. Bronze Tiger and Lady Shiva appear.
2014-2015
Flash Season One: Barry becomes the Flash in the first few episodes. The first season is basically the same, with the primary difference being that Wally is there from the beginning. Wally probably gets his powers around the midseason finale.
Constantine Season One: It happens. The whole season would've been completed though.
Arrow Season Tree: Oliver vs. Count Vertigo. Oliver meets Hal Jordan during the season. Laurel trains with Ted Grant and becomes Black Canary. Season ends with Oliver and Laurel leaving with Hal Jordan.
Crossover: Flash vs. Arrow. The Crossover made sence. Rainbow Raider and Captain Boomerang remain villains.
2015-2016
Flash Season Two: Flash of Two Worlds, Jay is actual Jay Garrick. Barry fights the Rogues this season. Earth-2 Wells is also introduced, as basically this show's equivalent of Johnny Quick. Wally meets Linda Park, and Cisco becomes Vibe this season. Season ends with Thawne returning for another fight with Barry, leading to Barry going back in time and creating Flashpoint.
Supergirl Season One: Close enough in concept. Max Lord turns out to be the main villain. Biggest change is that James Olsen isn't introduced. The Guardian is actually Jake Jordan. No Indigo. No Non.
Arrow Season Four: Oliver, Laurel, and Hal travel around the country adressing social issues. Many obscure Green Arrow villains are introduced. Sara Lance comes back from the dead. John Diggle goes and becomes a GL before season's end. Roy has his drug abuse storyline. The main villain turns out to be Damian Darhk. Laurel survives. Oliver meets Mia Dearden. Oliver decides to run for mayor before the end of the season.
Legends of Tomorrow Season One: Vandal Savage is the main villain. Hawkman and Hawkgirl would stick around. Captain Cold would die, only to be revived by Flashpoint.
Crossover: Millennium: Our heroes encounter the Manhunters during their attempted invasion of Earth. A couple characters are revealed as Manhunters. Barry, Oliver, Hal, Laurel, Kara, J'onn, Wally, and Roy are declared the "New Guardians." Kate Spencer becomes a Manhunter during this version.
2016-2017
Flash Season Three: Flashpoint happens, with the only real changes being that Ollie replaces Batman, and the WW v. AM subplot is erased. Barry comes back and things are similar, yet different. Caitlin still has powers in this version, but she doesn't become Killer Frost. Barry fights Future Flash in what is essentially a direct adaptation of the comic storyline. The events regarding the Future Flash break the Speed Force, causing Barry to go into the Speed Force at the end of the season.
Supergirl Season Two: Instead of the Daxamite Royal Family, we have Supergirl recruited into the Legion by Mon-El. The Fatal Five would be the main villains. Lena Luthor is also introduced. Maggie Sawyer doesn't leave by the end.
Arrow Season Five: Oliver is training a new team with Mia Dearden, Wild Dog, Ragman, and Mr. Terrific. Main villain of this season is Prometheus. Oliver discovers he has a son, Connor Hawke. Roy and Thea leave in the middle of the season. Season ends like in the show.
Legends of Tomorrow Season Two: Per Degaton would be the main villain, going after the JSA (Amaya Vixen, Hourman, Alan Scott, Ted Knight, Dr. Mid-Nite, Commander Steel). Carter and Kendra's connection to the JSA would be explored. Carter would die before the end of this season.
Crossover: Invasion! Alien races go to war. Dominators launch Gene Bomb. Scientists work to reverse it. Basically the comic story.
2017-2018
Flash Season Four: Wally is the new Flash. We also learn that in the months since Barry disappeared, the Rogues retired. The Samuroids show up, as do several minor villains. Barry's return is completely orchestrated by DeVoe in this version. Barry doesn't come back for a few episodes. In the background, we discover DeVoe is working with Thawne. Ralph also first shows up around here. Crisis on Earth-X happens, WestAllen marriage, but in the midseason finale, Barry kills Thawne. Trial of the Flash happens. When Barry gets out of prison, he fights a few more DeVoe created metas, and eventually works with Jay Garrick to defeat DeVoe, who's plan was ultimatly to test a mind control device on the city. At season's end, Nora shows up. Also, Iris is still a reporter in this version.
Supergirl Season Three: Reign the Worldkiller remains the main villain. Morgan Edge would also have an influence this season.
Arrow Season Six: Oliver and Laurel actually get married during the season. Main villain is Ricardo Diaz Jr. Over the course of the season, we would learn more about Richard Dragon, leading tonthe return of Lady Shiva and Bronze Tiger. Quentin Lance dies. Near the end of the season, Laurel and Lady Shiva would swap lifestyles for a few months.
Legends of Tomorrow Season Three: Mallus would be the main villains. Basic plot is the same, regarding the Time Bureau, Zari, and everything else. Wally doesn't join the Legends. Amaya leaves by the end of the season.
Crossover: Crisis on Earth-X! Nazi's from another world attack, causing the heroes to team up with Uncle Sam and the Freedom Fighters to liberate Earth-X. The Ray, Black Condor, Phantom Lady, Uncle Sam, and Doll Man are introduced.
2018-2019
Flash Season Five: The Nora plot is relatively similar, at least in the first half. The first half of this season involves Hunter Zolomon becoming Zoom, with almost 100% comic accurate motives. In a subplot, Caitlin's dad shows up. He's Icicle. He doesn't die this season. Elseworlds happens. Second half of this season introduces Amunet Black and the New Rogue's. Nora has her brief villain arc, but that ends pretty quickly. Nora actually survives the season.
Supergirl Season Four: Manchester Black survives. Agent Liberty is still the main villain. Lex shows up. Dreamer debuts. This season is basically perfect.
Arrow Season Seven: Oliver has his prison storyline and beats Diaz. Diggle and Lyla return during the season. When Laurel comes back, Sin is introduced. Oliver and Laurel battle the Night Circle this season, meeting Emiko Queen. Near the end of the season, Oliver makes a deal with the Monitor.
Legends of Tomorrow Season Four: Neron's plot is much close to that of Underworld Unleashed. Legends fight enhanced villains across the timestream. Martin Stein dies. Charlie is introduced.
Crossover: Elseworlds! Barry and Oliver swap lives due to Dr. Destiny. Barry, Oliver, Clark and Kara team up to fight Dr. Destiny, only to encounter 90s Flash and the Monitor. Dr. Destiny creates another reality, until Superman shows up and stops him, restoring reality. Oliver's deal with the Monitor is made.
2019-2020
Flash Season Six: The team fights Bloodwork in the first half of this season. This leads up to the Crisis, and Barry seemingly disappearing after his epic battle with Thawne (as teased in the pilot). The team works to bring Barry back in the second half of the season, while Daniel West shows up. The Eva McCulloch storyline works as it did in the show. Black Hole is also relatively similar, but they're also responsible for Daniel West becoming the new Reverse-Flash. Daniel dies and Barry returns by the end of the season. NuWally also makes his first appearence. The season ends with the Speed Force storm.
Supergirl Season Five: League of Ancients make for interesting villains. Lena wouldn't turn evil in this version. Kara would grapple with survivor's guilt from the Crisis. Guardian sticks around.
Arrow Season Eight: Oliver, Diggle, and Laurel travel around the Multiverse going on missions for the Monitor. We discover Lyla is Harbinger. Connor becomes Green Arrow during the Crisis. Oliver dies before series' end.
Legends of Tomorrow Season Five: Charlie's connections to the Loom are explored. Astra is released from Hell, as is Vandal Savage. Behrad Tarazi is introduced this season. The cliffhanger is the release of Black Adam.
Batwoman Season One: Elegy is properly adapted. More obscure Bat-villains!
Stargirl Season One: The show is basically the same, as season one was great.
Crossover: Crisis on Infinite Earths! Earths are destroyed with many cameos throughout. The heroes defend Earth-2, and save many lives. Earth-2 is destroyed. Oliver dies. The Monitor reveals the existence of Paragon, sending the heroes on various mission, while Pariah watches Earths die. Harbinger kills the Monitor. There's the huge heroic meeting, leading to the most powerful heroes from across the Multiverse going to battle the Anti-Monitor. During the battle, Smallville Supergirl dies. The Flash of Earth-1 dies destroying the antimatter cannon. Anti-Monitor personally goes and destroies the remaining Earths, but not before Pariah sends the Paragons (Wally, Routh Superman, Sara, Conroy Batman, Supergirl, Ryan Choi, Diggle, J'onn, and Lex) to the Vanishing Point. Oliver returns as the Spectre and helps the heroes gather the energy to get to the Dawn of Time, where they use the last remaining fragment of the Book of Destiny to empower Oliver to fight the Anti-Monitor. Oliver dies after Anti-Monitor is knocked out, and the universe is rebooted. The heroes then unite to battle the Anti-Monitor around the world, with Brandon Routh Superman landing the final punch. Brandon Routh Superman then goes into a pocket dimension, as the universe mourns the loss of Oliver and Barry.