Before Batman, Superman used to be the standard go-to character for gratuituous publicity in DC comics titles until around the late 80's-early 90's. He still occasionally appears as a guest to push up sales, especially when he always turns up in a flagging new series starring an untested character (of course, given Superman's speed he could actually pull this off if he wanted to). Supes acknowledges this trope in Sergio Aragonés' Fanboy where he says they usually stick him in the first issue of a new series to boost the sales.
'90s Anti-HeroLobo has also been used in this way. The Action Comics #695 cover had the strapline "In this issue: Lobo appears on one page!" As a parody of Wolverine, this is to be expected.
Justice: The second trade paperback has the Legion of Doom on the cover, with The Joker front and center. Except that the Joker was never in the Legion of Doom, wasn't broken out of prison when Legion member Scarecrow escaped from Arkham Asylum, and only makes cameo appearances in the rest of the series, in a Paper-Thin Disguise most of the time, and he blows up a building. None of those things have any bearing on the actual plot. The actual main villains of the series, Lex Luthor and Brainiac, are shoved off to the side behind Joker, as if to insinuate that they're supporting characters instead of the main antagonists. This may be a Mythology Gag in reference to the various Superfriends series, which Justice is heavily inspired by; Joker was originally slated to be a member of the Legion of Doom, but due to a Filmation cartoon being produced at the same time getting the rights to him first, the Ace of Knaves was largely left out ofSuper Friends.
WildStorm also tried to pull this off with Grifter, Zealot and, later, Midnighter, by having them make as many guest appearances as possible. Grifter and Midnighter actually starred in two miniseries just about them teaming up! Generally the best way to tell if a comic was in trouble was by how prominently Grifter appeared on the cover. When Wildstorm was still an Image imprint, Grifter and Zealot shared the spotlight with Spawn and Badrock. Badrock is an oddity in that his solo series was short-lived and he's much better known for Youngblood, but he was the star of the Marvel Team Up-like Badrock and Company and has had many crossover minis — including ones with Wolverine and Grifter.
Shazam!: Captain Marvel was often shoved on the cover back in the 40s, when he was the decade's most popular hero. Of course, since the number of genres was a lot more eclectic, you'd see him promoting the latest Funny Animal book or Western.
Forever Evil (2013): Though it is more or less par for the course, Batman's exposure in Villains Month far outshines most others. In all of his goings-on taking place in the present day, he has four villain issues, making for a total of sixteen between Batman, Batman and Robin, Batman: The Dark Knight, and Detective Comics, seventeen if Superman/Batman counts. The next highest amount is Superman at eight or nine following the same rules between Action Comics and Superman as his personal goings-on. The third member of the Big Three, Wonder Woman, has only two issues in her one ongoing, less than many of the other series going on. The Batman situation would not be so jarring but for the fact that some of the villains involved in it have no connection to Forever Evil (The Joker being highly publicized, but one of these), are very new to The DCU (the Court of Owls), are just being introduced at this time (Joker's Daughter) or are actually a rogue of another member of the Batfamily (Ventriloquist III).
Harley Quinn could be considered Deadpool's Alternate Company Equivalent in this regard. She's been a popular character for years, but became especially so after the launch of her New 52series and her role in the Suicide Squad (2016) movie. She appears in nearly all of the Batman: Arkham Series publicity despite her fairly minor role, is likely to turn up in almost any Batman-themed merchandise, and ended up as one of the most prominent characters in the DC Super Hero Girls line despite not actually being a hero. That's not even getting into her many comic book appearances, including two separate months where many books had Harley Quinn-themed variant covers (much like the Deadpool variants mentioned above).
The Jaime Reyes Blue Beetleactually got this a lot, with Co micBook/Superman, Batgirl and The Phantom Stranger showing up for a few pages and getting a picture on the cover of various issues. Though Vindicated by History, the series was never a huge seller when it was running, so this might well have been an attempt to promote it. It was never tooegregious, though, and is even kind of entertaining in the way that it makes Jaime seem like a Weirdness Magnet.
Due to the enduring popularity of Watchmen, DC has begun reprinting early work featuring Charlton Comics characters like Blue Beetle and The Question under the title Road to Watchmen, even though the extent of their connection to Watchmen was inspiring some of its characters.
While already an iconic character, the success of her solo movie led to DC marketing Wonder Woman as a much bigger deal, with the revised 5G timeline flat-out retconning her into having been the first superhero to emerge during the Golden Age. This extends to reprinted material as well, such as a paperback collection of the 90s Justice League America series being marketed as Wonder Woman and the Justice League America.
