Warning: SPOILERS for One Piece Film: REDA surprising scene in One Piece Film: RED gives credit to the theory that a huge villain will make her unexpected return in the future of the series. While the timeline of the movie is not necessarily canonical with the manga, the appearance of Big Mom, along with the Straw Hats' possessing powers and abilities that they obtained after her death in the Wano Arc, opens the way for the much-rumored return of one of One Piece's strongest villains.
Big Mom is one of the most iconic characters in the entire series. As one of the Yonko, the Emperors of the Sea, powerful pirates who rule over the New World, her appearance in the manga was teased for a very long time, and when it happened it did not disappoint. Big Mom is an impressive woman, possessing unbelievable strength and the powers of a truly dangerous Devil Fruit, the Soru Soru no Mi, which allows her to manipulate souls. If that weren't enough, Big Mom commands one of the largest countries in the New World and a fleet of pirates led by her powerful sons and daughters. However, all this might could not save Big Mom from being defeated and apparently killed by Trafalgar Law and Eustass Kid during the battle for Wano that took place in Onigashima.
The timeline of One Piece Film: RED is left vague, as is the case with every One Piece movie, so it's not clear if the events of the movie should take place before or after Wano. The appearance of the Big Mom Pirates, and of Big Mom herself, towards the beginning of the movie suggests that the festival on Elegia Island takes place before the Straw Hats arrived in Wano. However, during the movie, Luffy and his crew show powers and techniques that they acquired in Wano, including Luffy's already legendary Gear Fifth, which he learned during the battle against Kaido, and Zoro's new sword, Enma. At one point, Luffy also says he is a newly crowned Yonko, which happened after Kaido and Big Mom's defeat.
Big Mom's Appearance In Film: Red Should Not Be Dismissed As Non-Canon
While all this could be simply attributed to the fact that the One Piece movies usually don't care much about the continuity of the manga, it should also be remembered, however, that the events in Film: RED are strongly implied to be canon for the series. Luffy's childhood friendship with Uta, for example, or the new information on Shanks and his Red-Haired Pirates, should all be considered canon, which means that Big Mom's presence can't be so easily dismissed. Due to the circumstances of her defeat in Wano (falling into a lava pit while saying "You better not think that this is enough to kill me") fans have been speculating that the fearsome (ex) Yonko is not dead, and she will make a comeback at some point. Her appearance in Film: RED, then, could be a tease that Big Mom's time is not over yet.
When different media of the same franchise mix, the results are often confusing. Movies based on manga usually follow different timelines and are considered non-canon, but a lot of the appeal of One Piece Film: RED comes from the promise to reveal more about canon characters such as Shanks. In this context, the appearance of Big Mom along with the post-Wano Straw Hats in One Piece Film: RED cannot be so easily dismissed as non-canon, and it could instead be a hint that this unforgettable villain will make a comeback in the future.