Tomorrow is America’s birthday, so it only makes sense that Captain America might show up in Fortnite. Based on the video it seems he’s arriving via the Bifrost. The First Avenger is now available in the game’s shop — or, rather his outfit and two items are.
Right now, all players can pick up Cap’s Outfit along with his Proto-Adamantium Pickaxe (Cap’s Shield) and Back Bling. There’s also the Grand Salute Emote, which involves a lot of fireworks. You can see the emote in action on the Fortnite site.