Assuming some leaks and rumors are to be believed.
Fortnite Rainbow Royale

Several little snippets of leaks are adding up to suggest that Lady Gaga will be the next major concert event coming to Fortnite. The most recent piece of evidence pointing at this becoming a reality comes from a major Fortnite leak and datamining Twitter account, FNLeaksAndInfo.

On their Twitter is an audio clip for an emote named "Jug Band." Jug Band is one of those synced emotes that, as more people join in, creates a more cohesive song. The Jug Band emote was added to the game in the most recent 21.50 update. The song sounds like country/twangy version of Lady Gaga's song Poker Face. You can hear their findings below.

Epic Games also announced that their Rainbow Royale, an event that is dedicated to LGBTQ+ art, item drops, and other content would begin at 8PM (ET) tonight. Featured in this event are some LGBTQ+ favorite artists that includes, you guessed it, Lady Gaga. Players will be able to listen to Lady Gaga, and other artists like Janelle MonĂ¡e, Lil Nas X, Todrick Hall, King Princess, Kim Petras, and more on the in-game vehicle radio.

One last piece of evidence comes from the Fortnite Battle Royale subreddit. A post made earlier today seems to suggest that the Fortnite characters dancing in the Rainbow Royale promo image (seen above) are mimicking the dance moves Lady Gaga performs in her Bad Romance music video.

Nothing has been set in stone nor announced just yet of course. If Lady Gaga is the next major Fortnite concert, she will join other artists such as Ariana Grande, Travis Scott, Marshmello, J Balvin, and more who have also held in-game concerts.