In November 2018, the Japanese documentary show, Jōnetsu Tairiku, aired an episode focused on Hajime Isayama, the creator of Attack on Titan. In this show, Hajime said that he planned to finish the epic story in 2020. Internet speculation started to run rampant with everyone trying to predict how the series would end.
Though Isayama in on-course to remain true to his word, interestingly, the series has already been given an ending courtesy of the two live-action films. These films, entitled Attack on Titan and Attack on Titan: End Of The World were released in 2015. As the films were released before the end of the manga, the writers had to come up with an original ending themselves. In the process, they made a lot of other pretty drastic changes to the story.
One of these changes is a change in setting. While similar-looking to the setting found in the manga, the films explicitly state that they are set in the future. They're also set in Japan as opposed to Paradis Island. While these are small details, they do change how the audience perceives the location. The films also make some changes to the cast, removing some characters and adding some original ones. For instance, the human form of the Colossal Titan is Kubal as opposed to Bertholdt Hoover. Kubal is also the head of the Military Police Brigade, rather than Nile Dawk, who doesn't exist in these films.
Shikishima is one of the most important new characters. He is the stand-in for Levi and acts as a way to tell the audience about Titan physiology. However, while Levi is upset at the idea of killing humans, Shikishima is much more relaxed about it. In the movies, Shikishima is revealed to be Eren's brother. While Eren had no genetic siblings in the manga at the time this film was released, it was later revealed that Zeke Yeager was his half-brother, making Shikishima a stand-in for Zeke as well. Shikishima is also the White Titan, which is this movie's version of the Armored Titan, a title held by Reiner Braun in the manga.
The biggest change by far is the origin story of the Titans. In the live-action films, Titans are the result of a military experiment gone wrong. These experiments ended up unleashing a virus that turns all humans into Titans with no way of turning them back. This led to Titans overrunning and destroying everything in their wake. And the fact they don't need food and don't age makes them effectively invincible.
But it is the ending that really makes the movies stand out. A group of scouts aims to seal a gap in the wall with a large bomb, hoping that this will allow them to live in safety. However, they encounter Shikishima's troops on the way. Shikishima explains that he plans to use the bomb to pull off a coup, believing that if he blows up the second wall, the Titans will murder all of the common people in the city. This will force the government's hand, making them kill all the Titans, and then, after that war, Shikishima can pick off the remaining members of the government and install himself as ruler.
As Eren fights with Shikishima, Eren's friends escape with the bomb. Shikishima pursues the escaping group and reveals that he is the White Titan. He transforms into his Titan form and then goes to get the bomb. Eren transforms into his Titan form and fights with the White Titan. It is a close battle but Eren is able to eke out a victory. With the White Titan defeated, Eren puts the bomb in position and sets the timer. Eren returns to his human form, but he is very weak due to his ordeal. Just as the group relaxes, they are met by Kubal, the leader of the military police. He presents an ultimatum: Turn over Eren or die. Before Kubal can attack he is shot by one of Eren's group, causing him to turn into the Colossal Titan.
The Colossal Titan tries to grab the bomb, breaking it in the process. The others try to fight it, but they can't get the upper hand until Shikishima reappears. He transforms into the White Titan and declares that he will seal the wall. He does this by throwing the bomb into the Colossal Titan's mouth, causing an explosion that seals the wall. The film ends with the scouts looking out at the horizon, contemplating what lies ahead. However, at the end of the credits, we see a video of the wall being repaired. As the camera pulls back we see that we're in Shikishima's bunker, suggesting that he survived -- leaving the series on a frustrating cliffhanger.
While we don't know for sure how the main Attack On Titan series will end just yet, the live-action movies make enough changes to be considered a wild, alternative continuity.