It seems that the identity of Joker's stage in Super Smash Bros. Ultimate may have been revealed, as a user on Twitter has claimed to have datamined the game and discovered that Mementos from Persona 5 will be coming to Ultimate when Joker is released.
Masahiro Sakurai revealed during the final Direct for Super Smash Bros. Ultimate that five paid DLC characters would be coming to the game (not counting Piranha Plant, who was free to early adopters) and that each character would come with a new stage and a selection of songs. As of the time of writing, the only DLC character to officially be confirmed for Super Smash Bros. Ultimate is Joker from Persona 5.
Mementos being Joker's stage in Super Smash Bros. Ultimate has been a popular prediction since he was first announced for the game, as it has a unique aesthetic and it's somewhere that most players will spend a lot of time exploring when playing Persona 5, as it offers lots of side quests and will let you face monsters from the previous dungeons which become inaccessible as the story progresses.
Will Mementos truly be the Persona 5 stage in Super Smash Bros. Ultimate or is this datamine a hoax and Joker's stage is actually Cafe Leblanc? Will we actually be dodging pots of coffee thrown by Sojiro as stage hazards? We won't have to wait long to find out, as Joker is due to be available as a playable character in Super Smash Bros. Ultimate before the end of April.