It looks like James Gunn is going to take a different direction with post-credits scenes in the DC Universe than the Marvel Cinematic Universe has and fans should be happy for it!
The post-credits "stinger" existed for a long time before the MCU came along, as writers and directors enjoyed a post-credits bit that could either be a fun joke for the audience or tease a sequel.
The MCU, however, turned this into an art form, starting with 2008's Iron Man, as Tony Stark was met by Nick Fury to discuss "the Avengers Initiative." Ever since then, almost every MCU movie has ended with some sort of tease of the future plans, and those usually work out well.
James Gunn knows this firsthand. Guardians of the Galaxy ended with a light-hearted bit as the Collector was met by cult Marvel character Howard the Duck. Guardians of the Galaxy Vol. 2 had one confirming that Stan Lee's constant cameo character was a Watcher, while another set up the arrival of Adam Warlock. Vol 3 was different, with a scene of the new Guardians defending a planet and a quiet one of Peter Quill back on Earth.
The DCEU also had its post-credits scenes, some more successful than others. Justice League, for example, was meant to set up a sequel with the Legion of Doom, but that never happened. More famously, Black Adam had Henry Cavill making a return as Superman for a showdown that would be derailed when the DCEU ended.
As it turns out, anyone expecting such stingers to continue in Gunn's new DC Universe is in for a surprise!

Why the DC Universe is avoiding traditional post-credits scenes
James Gunn's Superman does have two post-credits scenes, but rather than set up major future plot threads, they're both low-key, with heart and humor in equal parts. Speaking to Screen Rant, Gunn explained that he wanted to avoid the "trap" of the MCU's stingers often failing to deliver the promised setups.
"Well, I found in my time at Marvel that, first of all, the post-credit scenes of mine at Marvel that people liked the best were the stupid ones. So, with Howard the Duck at the end of Guardians and Stan Lee at the end of Guardians 2. And I really have found – and I've said it before – but I've set up things in the post-credit scenes, and Marvel has set up things in the post-credit scenes that never pay off. Unless it's really for sure going to pay off... like if we were making – we know what the Supergirl movie is – So if it was a payoff at the end of the credits that had to do with something in the Supergirl movie, I would be open to that. We know that's happening. But I don't want to set up stuff just because it's a shocking post-credit scene that we're never going to pay off."
Gunn's point on the MCU failing to make their stingers work out is valid. He himself was trapped by this as Adam Warlock was teased in Guardians Vol. 2, but by the time the third movie was made, things had changed to alter the character arc.

While some after-credits scenes can work well (see how Spider-Man: Far From Home led directly to No Way Home), the more recent ones have been poor. Ant-Man and the Wasp: Quantumania was all about Kang, only for that entire character to be dropped. Eternals had the introduction of Harry Styles as Starfox and Mahershala Ali's Blade, with the returns of both characters now seemingly in limbo. While Thunderbolts* seems to tie in with The Fantastic Four: First Steps, the MCU stinger has lost its appeal.
The DCEU was in the same boat with bits like how Shazam! promised Mr. Mind, only for the sequel to ignore it completely. It really looks like the writers are just tossing in some bits to thrill comic book geeks without any concrete plans to use them later. That's especially true for Marvel, which has been in a constant state of flux since Endgame.
The thrill and joy these scenes once had has faded because of how it's a 50/50 shot they ever gets any payoff. Once, fans were more than happy to sit through the credits to get a peek at a future movie, but it now feels like a slog for some forgettable bit that becomes more unimportant as time goes on.
This makes Gunn's decision not to have Superman's end credits teasing more DC Universe plans much better. He knows full well that there can be anything from studio interference to other factors that cause these plans to derail, and holding off on things that may never come to be is much better. These films can stand on their own rather than being forced to include a lead-in for a new entry.
This may change as more DC movies are made and the studio opts to expand the universe. However, Gunn is on the right track that not every film needs these big stingers to wrap it up. If the DCU is going to buck the by-now-tired post-credits scene tease, that's a good thing for comic book fans.
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