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Since comics aren't as known as movies, they usually have more creative freedom. That means they can include storylines and address issues that never would have made it onto the big screen. The DCEU is a good example of this approach.
While the cinematic universe doesn't shy away from violence and sensitive topic such as death or an abusive relationship, it still takes care to be as approachable as possible for the majority of audiences. As a result, there are some DC comic storylines that are too much for the DCEU and never would have made it there, simply for being too edgy or frightening.
5 Joker Kidnaps Barbara Gordon
Batgirl has yet to show up in the DCEU. That's something that will change soon since the Batgirl movie with Leslie Grace playing Barbara Gordon is in the works. But even if Barbara crosses the path with the Joker, it's unlikely their meeting will go the same way as it did in the now legendary Alan Moore comic The Killing Joke. In the comics, the Joker kidnaps Barbara Gordon, and he eventually shoots her, leaving her paralyzed.
Even though Barbara does her best and becomes the superhero Oracle, the story is still too dark and brutal even for the DCEU that's mostly serious. What's more, it has been adapted into a movie before, the 2016 animated Batman: The Killing Joke, and it's unlikely the DCEU would want to do a story that's already been done, albeit not in a live-action movie.
4 Everybody Loses Their Powers
Not every DCEU superhero has special powers, but most of the iconic ones do. If they lost their powers for good, the situation could easily turn ugly. That's what happened in the comic JLA: Act of God. Once every powered hero loses their powers in this story, the villains use the news to their advantage, getting much more control than before. The heroes don't always take the loss well, such as Superman who starts drinking. If that wasn't enough, several superheroes die.
So even though a group of heroes pulls together in the end and defeats their enemies, it's too late to save everyone. The story not only works with several heroes absent from the DCEU which could be a problem but another reason why it will most likely never appear in the DCEU is that it changes the character of some heroes, portrays them in a way that the DCEU might not be so inclined to present.
3 Injustice Sees Superman Snap
Even though Superman is forced to kill in the DCEU, something some fans criticized, he never becomes as dark as his Injustice counterpart. In the storyline, the Joker decides that if he can't break Batman, he's going to break Superman instead. And it works. The Joker kills Superman's close friends Jimmy Olsen and tricks Superman into killing his pregnant wife, Lois Lane. The final nail in the coffin is the destruction of Superman's city Metropolis. Superman loses himself and kills the Joker. The story also gets darker from there, proving that sometimes even the heroes with the purest heart can break, seemingly for good.
It's unlikely that the DCEU would go in this direction since it would be very hard to reserve the deaths of all those who meet their end in Injustice. However, the fans who wish to watch the story in a movie should pay attention to the 2021 animated movieInjustice that's based on the comics. Even though it didn't get such high reviews as other DC animated movies, it's as close to a live-action adaptation as this comic book story will most likely ever get.
2 Characters Get Assaulted
The topic of assault on women is highly sensitive and relevant. The comics have addressed it multiple times, with mixed results. One such occasion happened when Sue Dibney, the wife of the superhero Elongated Man, was assaulted by Doctor Light. When the heroes realize what Doctor Light did, they find him and Zatanna used her powers to pretty much erase his mind, in a procedure gone long.
And when Batman questioned the treatment, Zatanna wiped his mind as well, albeit not with such a drastic result. As such, this storyline contained multiple controversial points that are unlikely to make an appearance in the DCEU anytime soon, if ever. Once again, the characters present in the story aren't yet in the DCEU, and even if they were, the DCEU has been careful to stay away from some topics, an assault being one of them.
1 DCeased
Marvel has its Marvel Zombies, recently partially adapted in the animated series What If...? And DC has the series DCeased that works with a similar topic but is even darker since it doesn't have the same dark humor Marvel Zombies possess. Everyone on Earth becomes very much like zombies after using their phones or other electronic devices. Suddenly, the heroes are desperately overpowered and even those who are usually strong and in control, die soon (such as Batman).
The series is so intense because the readers have no idea who will die and who will live, and the first category contains a lot more names than the second one. The only way this storyline could be adapted into a DCEU movie would be if the movie worked as a stand-alone but considering the connection between the DCEU movies, DCeased aren't likely to join the universe.