Over the past couple of years, Fortnite has played host to a variety of big crossover events, promoting Hollywood films and even other video games. Fortnite crossovers have included events based on Avengers, John Wick, and most recently, Stephen King's It. Fortnite crossover events have been available on all platforms at the same time, but it seems like that may change with a future crossover event that has potentially leaked.
According to prominent Fortnite dataminer HYPEX, evidence has been found in Fortnite hinting at Splatoon cosmetics for the games. More specifically, the files point to a Splatoon-themed skin and backbling. Since Splatoon is a Nintendo-owned IP, it stands to reason that this content would only be available on the Nintendo Switch version of the game, and not on PS4, Xbox One, or elsewhere.
The extent of the potential Fortnite and Splatoon crossover is unclear at this time. It's possible it will literally consist of just these two cosmetic items, or it could be a more fully-featured event like we've seen in the past, complete with its own unique game mode. Time will tell, and at this time, it's important to note that nothing official has even been announced and the crossover may not even happen at all.
Since Fortnite started doing crossover events, most of the crossovers have been for movies instead of other video games, so it would be somewhat surprising if the Fortnite/Splatoon crossover comes to fruition. That being said, the recent Fortnite/Borderlands crossover was actually rather extensive, featuring a whole area on the battle royale map that was made to look like Borderlands' cel-shaded art style, and even featured a mission involving Claptrap.
If the Fortnite/Splatoon crossover event is real, it seems likely that it won't be quite as extensive as the Borderlands crossover. After all, Splatoon is a Nintendo-owned IP, so it likely wouldn't be able to be featured on all platforms. However, it wouldn't be out of the question for there to be cosmetics that players can only earn in the Switch version of Fortnite.
Then again, Fortnite's cross-play functionality (the Switch has crossplay with Android and iOS mobile devices) may throw a wrench in that as well. There's really a lot of different possibilities when it comes to the Fortnite/Splatoon crossover, so fans will just have to wait for an official announcement.
Fortnite is out now in early access for Android, iOS, PC, PS4, Switch, and Xbox One.