Fantastic Four: 10 Theories For How They Can Join The MCU
1. Filling The Void Left By The Avengers
When Marvel Studios introduced Tom Holland's Spider-Man, they effortlessly established the fact that the teenage Peter Parker had been fighting crime under the radar from The Avengers. As a result, there's no reason to suspect that the Fantastic Four couldn't have been part of the Marvel Cinematic Universe for at least a few years.
As mentioned, the team could have gained their powers during the first Snap, only to have remained hidden in the five years that followed as they attempted to master those newfound abilities (and Reed Richards tried to cure his best friend Ben Grimm of being "The Thing").
However, with Iron Man dead, Captain America M.I.A., and the Avengers disassembled (again), the Fantastic Four could realise that they need to step out of the shadows and become superheroes, thereby filling the void left by Earth's Mightiest Heroes. This would be a great place to pick up with them, and watching them come to terms with being celebrity superheroes would be great fun.