Epic Games and Marvel seem to have developed a taste for cinematic Fortnite crossovers and another is coming in a few days. You're looking at the poster for Avengers: Endgame in Fortnite, although details are pretty scarce at the moment.
What we do know is what Epic gave us, which is that it starts on 25 April 2019 and that it's got something to do with Avengers.
As you can see, the Fortnite character on the poster is holding Captain America's shield, so it's safe to assume it will have something to do with it.
Epic's first Fortnite-Marvel crossover was quite a successful one, as Thanos came out swinging, in spite of a number tweaks it required to be a proper and fair fun.
The mode was named after Thanos' soul gem equipped Infinity Gauntlet, and whoever picked it up became him until dead or a winner. There was that bug that allowed him to construct and use weapons, but overall - Fortnite's Marvel mode was a blast.
Hopefully, the new Fortnite-Avengers crossover ends up in the same vein, although you could argue that playing with the Infinity Glove sounds better than Captain America's vibranium shield.
On the other hand, it's probably wiser to wait and see what Epic have in store before making any judgements, because gameplay wise - Cap's shield might as well turn out to a much better addition to Fortnite.
Game designers are well aware that simple systems tend to work wonders in adding layers of complexity and fun, without diluting existing gameplay. Moreover, no bug will let you prance around as a juiced up titan that's too OP to stop, so yeah, bring in the Captain and his shield.
For all the Fortnite-killers we've seen launched in the past year or so, no game seems to be coming close to touching Epic's dominance, and the streaming dip they've had around Apex Legends seems to have been a temporary one.
You can find Epic's tweet with the date and the teased image over here.