Court documents from Epic's battle with Apple first suggested Grande would be coming to Fortnite this October (with Lady Gaga taking the Party Royale stage in December), and the leaker now suggests that Epic has completed an in-house test show. These shows would run in-line with previous concerts featuring Travis Scott and Marshmello, which have only gotten more bombastic with time.
The leak also suggests an imminent Stranger Things crossover adding something called "The Sideways"—a realm that, like the show's Upside-Down, will spill horrible monsters into the game world. Epic also plans to add a number of Justice League and Suicide squad characters, and is trying to get access to Naruto for Season 8 alongside an explosive kunai weapon.
The court documents are obviously legit, while the leaks come from a source the Fortniteleaks moderators claim has a consistent track record (including Fortnite's "Primal" season) and is "100%" trustworthy. There are a smattering of other teases not floated in court documents—including map changes, the return of rideable mechs and a complete map change for Chapter 3.
As with all leaks, it's worth taking these with a pinch of salt. But with the Grande concert and Naruto tease appearing in the official documents, such confirmation suggests the unofficial leak is accurate too.