Ever since the start of the Marvel Cinematic Universe's Phase 4, the question on everyone's mind has been what the next Avengers-level threat will be. While many plot threads are being put in place, the likely culprits of this would be Kang the Conqueror or an Incursion leading to a Secret Wars event, but those threats are as high, if not higher, than the Infinity Gauntlet. That said, it's likely that the next threat to hit Earth would have to reunite the Avengers and also brings a light, cosmic level threat to the picture. Therefore, this would imply adapting a version of the classic Avengers comic story, The Kree/Skrull War.
In the comics, this storyline was spearheaded by Roy Thomas, Sal Buscema, Neil Adams and John Buscema. To be fair, the storyline technically began long before the event from Avengers #89-97 and continued long after these issues. However, the events that did occur in these issues were important for everyone involved and tied up many plot threads, while kick starting others.
The Kree/Skrull War chronicled both alien empires as they finally brought their war to Earth since the planet was a strategic advantage for whoever conquered it. The only opposing forces on the planet were heroes like The Avengers, who had already been infiltrated by Skrulls and had a negative public image. While not verbatim, an MCU account of these events has already been set up in multiple stories.
The Kree Empire has oppressed the Skrulls for years, leading to a longstanding war between both races. In the MCU, this was established in Captain Marvel, revealing that not all forces on both sides are inherently evil. Spider-Man: Far From Home then confirmed that there were Kree sleeper cells on Earth, while Nick Fury was now in space working with some Skrulls. The final piece of the puzzle may come with the Secret Invasion TV series that could see the Skrull Empire trying to take over Earth before the Kree, nearly leading to an all-out war.
With Shang-Chi and the Legend of the Ten Rings and Ms. Marvel teasing ancient artifacts, with the latter's bangle being attached to a blue-skinned humanoid, The Kree may have also set up their defenses centuries earlier. Plus, the Ten Rings have already teased a beacon being sent, perhaps to a larger armada signaling Earth is ready for invasion. With all of these moving parts, the final confrontation would be both Empires facing off on Earth with a rag-tag team of veteran and new heroes stuck in the middle. While The Marvels may tease Captain Marvel, Monica Rambeau and Ms. Marvel getting ahead of the invasion, they may be too late to stop what's coming.
On Earth, The Avengers are still beloved, but their long absence may wear thin as aliens begin to destroy homes and attack innocents. Therefore, their public opinion may drop so that when these heroes unite, they don't have the same support as before. This would leave them to fight an uphill battle against enemies and those that lost faith in them, similar to the comics. Through all of these trials, a new team would emerge, stronger than ever, with the full support of Earth and a mild at best respect from other galactic Empires.
The Kree/Skrull War would easily be the most complicated story to adapt from the comics due to its longevity and impact. That said, with so many pieces in place, a film establishing that this could be the first of many invasions in the future would only benefit the MCU. For starters, it would open the stage for more cosmic characters to enter and leave Earth, like Nova. Second, it would put the planet on constant alert, further meriting a new version of Avengers that must remain on call and not only appear when there's a crisis. All in all, it would also bolster the ranks of the team, preparing them for whatever the next Avengers-level threat is.