As mentioned elsewhere, the Big Two comic book companies have a stable of characters with proven staying power, culled from decades of trial and error. But sometimes they want to launch a new character for various reasons. New characters tend to fail due to a combination of Sturgeon's Law and fandom's attachment to the existing vetted characters.
Legacy Characters do inject fresh blood into a title sometimes but they only revitalize the existing franchises rather than creating new ones. The solution is the Legacy Launch. It works something like this.
- Trope Man aka Bob retires or is killed off.
- David steps up as the new Trope Man.
- After a fandom has developed for David, Bob is brought back.
- David graciously abdicates the Trope Man identity back to the original.
- David assumes a new superhero identity of his own and his popularity (hopefully) endures.
Quite often, the new identity will have a lot in common with the old one. Similar powers and tools, usually with a different personality. This character can sometimes be a Suspiciously Similar Substitute, or Expy, or a Foil for the original. Younger and Hipper and Darker and Edgier seem to coincide with this trope too, probably because the original heroes are Silver Age and Golden Age creations.
The above steps are sometimes not intentional. For example, Bob could be killed and replaced by Dave to boost slumping sales only to provoke backlash from the loyal fans, so the old character is brought back only to provoke a similar backlash from David's new fandom making the creators turn to this trope as a compromise.
- Iron Man Tony Stark is replaced by James Rhodes. When Stark returns to the role, Rhodey becomes War Machine, which is basically Iron Man with More Dakka and a military background.
- Thor was bonded to another mortal temporarily. When Thor was separated and banished, the mortal, having been valorous and physically dependent on the joining, was given his own powers to become Thunderstrike.
- Captain America:
- At one point, Steve was forced to surrender his identity to the US Government, and it was then given to John Walker. When Steve got it back, John became US Agent.
- Inverted with the Nomad mantle: It was a new identity Cap created for himself, which was then passed down to some of his sidekicks and related characters once he went back to being Cap.
- Later, Steve was killed, and his old sidekick Bucky Barnes, formerly the Winter Soldier, became the new Captain America. When Steve came back to life, he let Bucky stay Cap, but later, due to a series of events, took on the mantle again, and Bucky was back to being Winter Soldier again.
- When Steve was incapacitated again by being depowered, he handed his shield over to The Falcon, Sam Wilson, his long standing sidekick. After he got his youth back, the two operated simultaneously as Captain America with Steve using a new shield. When it turned out the young Steve was actually a cosmic cube created imposter, the real Steve was restored by that same sentient cosmic cube and Sam gave up what he considered to be a tainted mantle. So Sam got his first ever ongoing as The Falcon, after two series as Captain America.
- John Henry Irons was introduced as a potential Legacy Character for Superman, but the plan all along was for Superman to return and Irons continued on as "Steel" with only the S-Shield missing (though he was already very different from Superman.)
- Jean-Paul Valley was briefly Batman but went crazy forcing Bruce to return to the mantle. After rehabilitating, Valley became Azrael.
- Hank Pym has inverted this, having created a number of identities for himself, Giant-Man, Goliath, Ant-Man, and Yellowjacket because he was never quite happy with them but has allowed other characters to use these identities (including Hawkeye who already had his own identity.)
- Marvel temporarily retired King T'Challa, the original Black Panther, and passed the mantle to a more "relatable" character named Kasper Cole. The final arc of Christopher Priest's Black Panther run saw T'Challa reclaiming the mantle and Kasper becoming the new White Tiger instead.
- Daken got his first solo series when he briefly stole the Wolverine identity while part of the Dark Avengers. Once that series ended, Daken received his own solo title without the Wolverine costume.
- Simon Baz, the newest Green Lantern, was introduced amid much fanfare about DC having a Muslim superhero. Geoff Johns made Simon the lead in his final year of Green Lantern, before returning the book to Hal Jordan and placing Baz in the new Justice League. He still uses the GL name, he just no longer has his own solo book.
- Two other Green Lanterns have gotten their starts as Hal Jordan's backup or substitute before becoming full time Lanterns in their own right. But Guy Gardner best fits the trope as he eventually went on to helm his own title wielding a yellow power ring and later still became The Warrior with entirely different powers.
- Monica Rambeau became Marvel Comic's Captain Marvel for a while but eventually took on her own name as Photon, and later Spectrum.
- The Clone Saga was originally planned to be this. Peter Parker was to discover that his long thought dead clone had gone on to live another life as Ben Reilly. Then Ben was to be revealed as the true Peter Parker with our Peter Parker being the clone, with Ben then becoming Spider-Man for a while. Then it would be revealed that Peter was the real Peter Parker with the hope that by then Ben would be popular enough to launch his own series as the Scarlet Spider.