Super Smash Bros. Ultimate on Nintendo Switch has by far the biggest lineup of fighters and stages in Smash history, but Nintendo isn't stopping there. At least six DLC characters (including the recently-released Piranha Plant) are planned, and five will come with their own stages. The next DLC fighter to drop is Persona 5's Joker, and based on some code discovered by data miners, we have a pretty good idea what his stage will be.
After digging around in the game's code, data miner Dr. HyperCake has discovered the phrase "Jack_Menentoes_" in association with stage name strings. "Jack" was the code name for Joker discovered in the data weeks back, which makes it likely that this is referring to the Persona 5 stage Mementos, also known as the Prison of Sloth or the Prison of Regression.
Obviously, this does not constitute an official announcement, and it's possible that a code name could be intended to hide the truth, but thus far data miners have been fairly spot-on with predicting upcoming content based on the code. We'll just have to see how this one plays out when Joker launches, which should happen by the end of April.