Some fans really love thinking about these scenarios.
Sometimes anime fans wonder how their favorite series would work if all the characters' genders were switched. It's an interesting theoretical scenario, but it rarely happens outside of gag comedies and April Fool's jokes.
Still, it's fun to think about — and here are some series that wouldn't work with genders swapped (as well as some that would, of course).
Wouldn't work: Vinland Saga
As harsh as it sounds, Vinland Saga simply wouldn't work if the protagonist — and practically the entirety of the rest of the cast — were female.
The anime is praised for its realistic portrayal of medieval times, in which female soldiers were extremely rare — and women were excluded from most positions of power. As such, Vinland Saga based around female characters would be much more fantasy-like.
Would work: Kaguya-sama
If you flip the genders in Kaguya-sama: Love is War (Kaguya-sama wa Kokurasetai: Tensai-tachi no Renai Zunousen), you'd get… the same anime, basically.
Seriously, the entire premise of the anime is that two characters want to make the other confess, at first. Both get equal amounts of screen time, and both are treated equally. If they were switched, nothing in particular would change.
Wouldn't work: Don't Toy with Me, Miss Nagatoro
As is, Don't Toy with Me, Miss Nagatoro (Ijiranaide, Nagatoro-san) was already criticized. The relationship between the two main characters is literally bullying, at least in the first episodes. It gets closer to friendly (and even romantic) teasing later, but the first episodes were enough to start a controversy.
Now, if the genders were switched, the anime would have caused an even more massive public outcry. It would border on abuse, and that definitely doesn't make for good anime material.
Would work: Free!
Okay, to be fair, while Free! would work if it were about a cast of girls, there's just one catch. As is, the anime is mostly popular with female audiences — and sports fans. It's often said to be one of the few anime where fanservice usually focuses on male characters.
If it were genderswapped, the cast of female characters in a swimming club would probably turn the anime into an ecchi series, basically. So it would work, but the target audience would be completely different.
Some fans even argue it works the opposite way as well: ecchi anime, genderswapped, would be just fine, but they'd be appealing to a different audience.
Wouldn't work: Blue Lock
On paper, there's nothing wrong with Blue Lock's premise. We have quite a lot of anime which focus on female sports, and while soccer isn't among the most popular ones, it's definitely normal enough to get an anime about it.
The issue with Blue Lock (female version) stems from a completely different problem. The whole idea of the training program in Blue Lock is to make a star team for Japan to win the World Cup. Except… the female soccer team of Japan has already won it, so the premise would have to be changed entirely.
Would work: Demon Slayer
Demon Slayer (Kimetsu no Yaiba) is a battle shounen, yes, and most of those have traditionally followed male protagonists.
That said, female protagonists for the demographic have been getting more popular lately. While not technically a battle shounen — but still a shounen — The Promised Neverland (Yakusoku no Neverland) is a good example.
It's hard to imagine why Demon Slayer with genderswapped cast wouldn't work, especially considering that its themes are practically universal.
Wouldn't work: Lovely Complex
Lovely★Complex used to be one of the most popular shoujo romantic comedies out there. It's still talked about every now and then, but the anime simply wouldn't work if the characters' genders were swapped: the premise basically requires it to work as is.
Both main leads aren't exactly happy with their height. Risa thinks she's too tall for a girl, while Atsushi thinks he's too short for a boy. If it was the other way around, it'd make no sense.
Would work: Unlimited Blade Works
This is the case of the genderswapped version already being a thing. Well, very technically. In the first drafts of the Fate/stay night visual novel, the protagonist was actually a girl, Ayaka Sajou. Similarly, Saber — Arthur Pendragon — wasn't genderbent in that draft. It was later adapted into short stories and an OVA called Fate/Prototype.
Still, some fans would actually want to see an entire series based on the premise — and with animation by ufotable, as was the case with Fate/stay night: Unlimited Blade Works.
Wouldn't work: Dance Dance Danseur
Dance Dance Danseur is one of the few anime that deal with gender roles specifically. As such, it's understandable why it simply wouldn't work if the protagonist wasn't who he is.
Junpei Murao has to keep his love for ballet a secret — as it is perceived as something effeminate, unbecoming of a man. Exploring that aspect and the social stigma behind it is a part of the anime that can't really be taken away from it.
Would work: Laid-Back Camp
"Cute girls doing cute things" is a genre that has its fair share of fans. It usually overlaps with slice-of-life. Still, there's no reason for the genre of "cute boys doing things" not to exist. In fact, it does, but it's much less widespread — and the audience is, predictably, very different.
However, Laid-Back Camp (Yuru Camp△) is considered one of the best CGDCT series out there because it's just comfy and calming — that part wouldn't change even if it were about boys.