Fortnite is starting an event on April 25 as a crossover with Avengers: Endgame, as evidenced by a couple tweets by the official Fortnite Twitter account. There is some speculation as to how many new items are showing up during the event, however Captain America's shield and Thor's new hammer seem to have been confirmed through the tweets by Fortnite.
This isn't the first time Avengers has done a crossover with Fortnite; in fact, it's the second. The first was a cross-promotion for Avengers: Infinity War that ran from May 9 to May 15 of 2018 where you could find and use the legendary Infinity Gauntlet. Following the event, the item was slightly nerfed to retain balance.
Season 9 of the popular free-to-play battle royale is expected to release following the crossover event on May 9.