DJ Marshmello is well known for titles like Alone, Happier and Together but also for being a video game fan and a Fortnite player. He also won the pro-am tournament alongside Ninja.
Two elements point to an involvement of the DJ in Fortnite. First, Moe Shalizi, Marshmello's agent, shared a Instagram story in which he implied a partnership: "Something is coming with Mello and Fortnite next month that's gonna have the world shook."
Secondly, there are those files discovered by lucas7yoshi in the 7.20 update. Entitled "FortniteGame\Content Films\\FestivusVideo5MinTest 02.mp4.", they hint at a "Festivus" event that could be in preparation. Furthermore, the file in question is a video and a remixed version of his title "Wolves" (featuring Selena Gomez).
So all that remains is to see how Epic Games intends to set this up and more importantly when? Maybe for the launch of season 8?
Are you interested in an in-game live concert in Fortnite?