WARNING: The following contains spoilers from the Demon Slayer: Kimetsu no Yaiba manga, by Koyoharu Gotouge, available in English through VIZ Media.
As the long-awaited second season of Demon Slayer approaches, the series will pick up where Mugen Train left off with the Entertainment District Arc. Considering this storyline will likely take up the bulk of Season 2, the following arc, Swordsmith Village, would be the perfect story for a second Demon Slayer movie. The success of Mugen Train proves that canon films can work, which is perfect because the Swordsmith Village Arc would be much better off as a movie rather than potentially being used for the third season.
There's no sense in doing a second movie just to rehash what the first one did well, which is why the Swordsmith Village Arc is perfect as the next, hypothetical, movie because it is very different from Mugen Train. This storyline ups the ante more than ever, as Upper-Ranks Four and Five of the Twelve Kizuki launch a coordinated assault on the titular village. Even though there are two Hashira present, they are not there just to fight on Tanjiro's behalf. This time around, Tanjiro spearheads the battle against the invading Upper-Ranks, with the Hashira struggling right alongside him.
Helpfully, the Swordsmith Village Arc takes place in one location and features a fixed number of characters, so there wouldn’t be too much for viewers to keep track of during a film. Swordsmith Village spends a lot of time exploring supporting characters, in addition to Tanjiro and Nezuko. The Hashira were all pretty cartoony during their introductions because the series didn't have much time to spend on them. However, Kyojuro Rengoku's portrayal in Mugen Train establishes that the Hashira each have the potential to be very rich characters worth exploring.
The Swordsmith Village Arc focuses on two more of the Hashira, Mitsuri Kanroji and Muichiro Tokito. Kanroji, the Love Hashira, explains to Tanjiro that she joined the Demon Slayers to find someone to marry. In her hometown, she repulses men with her absurd strength, as her muscle density is far above the average human's, making her much stronger than she appears. Tokito, the Mist Hashira, is also revealed to have lost his memories from his childhood, memories that slowly return to him during his battle with Gyokko, Upper-Rank Five. These two Hashira engage in ferocious battles with the Upper-Ranks that would be best suited to the biggest screen possible.
This arc also does a lot for Genya Shinazugawa, who has been around since the beginning of the series but only seldom appears in antagonistic roles. Revealed to be the younger brother of Sanemi, the Wind Hashira, Genya pursues strength because he wants to prove to his older brother that he belongs in the demon Slayer Corps. Despite being unable to perform Breathing Techniques, shockingly, Genya possesses the ability to take on demonic powers by consuming the flesh of demons. Genya's character develops quite a bit in a short amount of time, making it perfect for a film format, whereas his growth would feel less significant if it occurred in one or two episodes amongst many others.
One of the biggest reasons that the Swordsmith Village Arc would be better off as a movie is to help compensate for the absences of Zenitsu and Inosuke. Tanjiro's friends and fellow demon Slayers are not present for the entirety of this arc in the manga. As central characters and fan favorites, their absence would draw much less attention in a two-hour movie. Meanwhile, their absence would end up being a distraction in a weekly, episodic release spanning several months, with viewers wondering which episode will feature their return.
At first glance, a demon attack on the swordsmiths could easily look like a low-stakes affair, but this arc certainly does not lack for intense character drama. Tanjiro's worst nightmare becomes reality when he is forced to choose between saving Nezuko from the rising sun or pursuing Upper-Rank Four who is racing toward the villagers to consume them before escaping. Unable to decide, Nezuko kicks Tanjiro toward the demon, which he takes as his cue to choose the greater good over his sister. Tanjiro ultimately defeats the demon, but not before Nezuko is exposed to the sun's lethal light.
The Swordsmith Village Arc is not only a turning point for the entire series, but it also sets the series' endgame into motion. In a game-changing twist, Nezuko does not burn away in the sunlight, officially becoming the first demon in history to overcome this fatal weakness. However, the celebration is short-lived as the series cuts to Muzan Kibutsuji, who has sensed Nezuko's survival and realizes she is the key to his own conquering of the Sun. The arc ends with a dramatic cliffhanger worthy of the big screen: Muzan coming for Nezuko.