Heroes Unlimited

A Story Arc or season which features an expansion of the general cast, such as other groups of protagonists working in other locations for similar goals. Allows a new pool of characters for writers to pick from rather than constantly having to make new ones, so stories can occasionally focus on them and give the main folks a break. Often, characters from one-shot episodes are brought back as part of the new group.

Also useful as subtle Filler and potential Spin-Off material. Can very easily lead to Loads and Loads of Characters. See also The Chosen Many, West Coast Team.

Nothing to do with the Palladium Books RPG of the same name. Even though that first turned up twenty years before Justice League Unlimited.

Anime and Manga

 * The "World Tour" arc of Digimon Adventure 02.
 * Taken further in Digimon Tamers, though played more realistically.
 * The foreign Gatekeepers in Gate Keepers.
 * In the world of Pokémon Special, if you somehow get a Pokedex (gift, theft, accident, doesn't matter), it is a guaranteed that you're a necessary force in combating the evil plaguing your region for whatever reason, coincidence, or personal status. Three Dexes are made for each of main regions (with Yellow receiving an older model when the Kanto trio got their upgrades), so with the fifth generation underway, the numbers should logically go up to sixteen. For now.
 * Also, each set of regional Gym Leaders officially forms a defense group against whatever evil terrorist group is around. Sure, the Holders always save the day, but the Leaders usually provide very efficient backup.
 * Happened three times Bleach:
 * Soul Society arc introduced the 13 Shinigami Squads, each represented by a captain, a lieutenant and sometimes several lower-ranked members.
 * Zanpakutou Tales filler arc showcased physical manifestations of Empathic Weapons of most of the cast.
 * The fullbring arc in manga introduced a number of supernaturals similar to Inoue and Chad.
 * In Naruto, the Chuunin Exam arc introduced, in addition to the Rookie 9 (the classmates of the main characters), Team Gai from the year above, Team Sand from the Sand village, and Team Sound from the Sound village, and Kabuto and his teammates. Each team consists of three Genin and one Jounin sensei, making a total of 24 new characters. And that was only the beginning of the arc, and doesn't include Big Bad Orochimaru, Special Jounin like Ibiki, Anko, Hayate and Genma, Jiraiya, and the Sound 4. To be fair, a handful of these characters died. Also, the Anime tones this down slightly by introducing some of Naruto's classmates in the first episode. The Chuunin Exam is only the second story arc.
 * Magical Girl Lyrical Nanoha As doubled the size of the main cast compared to The Original Series. Then StrikerS came along and doubled the size of the entire cast compared to the second season.
 * Axis Powers Hetalia gets bigger with Hetalia World Series. Enter the rest of the world!

Comic Books

 * The All-Star Squadron, which is essentially Justice Society Unlimited set in the 1940s.
 * The Legion of Super-Heroes.
 * Warren Ellis's Global Frequency is like this from the start. The only characters who recur regularly are Aleph and Miranda Zero.
 * The proposed TV adaptation would have added two extra regular characters, Sean Flynn and Katrina Finch, who would join up with the rotating cast of each episode. Only the pilot was ever made though, so they're both one-shot characters after all.
 * Averted with the X-Men, while their roster is large enough, they're usually split into teams with each team having its own book. The roster mostly melded together when they moved to Utopia in 2009. This, coupled with some new additions to the cast, actually created a bit of a character overload.
 * Birds of Prey always had a very large cast of supporting characters in its early issues, but under Gail Simone the core team was Oracle, Black Canary, Huntress, and Lady Blackhawk. Once Black Canary left the series, however, the cast became a bit more fluid, with a rotating lineup. Gypsy began appearing just before Canary's departure, but slowly faded out once she left. Judomaster, Big Barda, and Hawkgirl appeared for individual missions, with Misfit (an original creation) joining as a new regular in the series (though she was never really on the team). Manhunter became a recurring supporting character, and they were joined by Infinity (another original character) shortly before the series was cancelled. A dozen other heroines (from Power Girl to The Question) are shown as receiving invitations to join the team or having been active in past missions. After the series was relaunched in 2010 the cast stabilized at six members: Oracle, Black Canary, Huntress, Lady Blackhawk, and Hawk and Dove.
 * The number of Avengers (including honararies) is truly staggering. It's sort of a good thing a lot of Marvel heroes don't work and play well with others, as what they could accomplish if they ever stopped fighting each other could be incredibly frightening.
 * The roster only increased with the Avengers Initiative.
 * While there's usually less than a dozen active members on the main Avengers team (or "teams", at times) at any given time, most former/reserve members are always welcome to come back. And when the situation gets really desperate, it's common for them the old members to show up.
 * Buffy The Vampire Slayer Season Eight has the Scooby Gang leading an army of hundreds of slayers, not to mention a large group of wiccans and other magic users.

Fanfic

 * A very common trope in fanfics based on adaptations from other subjects. For example, X-Men Evolution fic based after the series end? Expect either introducing comic characters every chapter, a big group of comic characters added as a new team, or any number of comic characters to be made regulars right from the first chapter with no introduction. Then, there's big OC fics...

Live Action TV

 * Lost tends to do this each season.
 * The second season added the characters from the tail section of the wrecked plane to the cast.
 * The third season added Desmond and a few of the Others to the main cast.
 * Season four introduced the crew of the freighter.
 * The fifth season
 * Heroes made this their policy from the very beginning.
 * The last season of Buffy the Vampire Slayer.
 * House did this in its fourth season. They got a whole new team for House but at the same time kept all the previous regulars as well.
 * Fringe did this with the alternate universe's Fringe Division.

Literature

 * The Animorphs significantly expanded their membership towards the end of the series.

Video Games

 * The purpose of Mass Effect 2, which involves Shepard recruiting a team of mostly-new Badasses on a suicide mission.
 * City of Heroes, as the name suggests.

Web Comics

 * Axe Cop routinely holds "tryouts" for new superheroes to join his team whenever a significant threat presents itself. Then there was the time he joined up with Bat Warthog Man's team.
 * Heroes Unite is this for all webcomics related to it.

Web Original

 * The Global Guardians PBEM Universe started off with a single campaign of only seven lead characters, plus a handful of supporting characters. Fast forward ten years and there are nearly twenty campaigns, nearly four hundred lead characters, and over ten thousand background characters, supporting characters, former lead characters, and villains.
 * The Whateley Universe has grown like this. The original six main characters have grown to around 20 main characters and another dozen or so who have gotten A Day in The Limelight or Character Focus or just a lot more facetime than originally planned. This is only possible because the original half dozen (or so) canon authors have grown to over twice that.

Western Animation

 * Justice League Unlimited, naturally.
 * The Avalon Tour arc of Gargoyles, where Goliath and the others happily discovered the other gargoyle clans still surviving in the world.
 * And again in the comic books: "Clan-Building" ends with the main cast nearly doubling in size, as not only do Coldstone and Coldfire join, but Brooklyn goes on a time trip and comes back with a wife, a kid (with another on the way), and a "dog".
 * Teen Titans (Titans East, and most of the 5th season.)
 * As Codename Kids Next Door progressed, it was revealed that the KND was a massive global operation. And despite the main characters being assigned operative numbers numbuhs 1 through 5, they were not at the top of the command structure.
 * The first season of Young Justice focused mainly on six or seven young super-heroes, although it did occasionally look at their mentors in the Justice League. Season two,, introduced eight new characters in the first episode alone), giving this type of feel.
 * Season one also ended with multiple solo heroes being inducted into the Justice League, significantly increasing its size. Season two has many of the Young Justice characters having been promoted to the Justice League.
 * The second season of Avengers Earths Mightiest Heroes has steadily been introducing more superheroes, and based on this screen shot (spoiler warning), it looks like the Avengers will be getting a lot of new members.