Man, I wish Secret Invasion had been used as a premise for, like, Avengers 4. Release it after The Marvels.
SPOILERS FOR BOTH
Giah (Talos's daughter) is shown in The Marvels as working for Fury on SABER, being friends with Monica, and being anxious about the Skrull-Kree peace talks. After the movie's climax, briefly show her arguing with Fury, blaming Danvers for Monica's death, and - when she finds out that Danvers actually went to help the Kree fix their sun, she quits, furious.
In a post-credits scene, show the Skrull emperor Droge recruiting Giah. He asks if she'll help them found a new Skrull colony, but (with ominous music playing) she says that Earth is her home - and now it will be her people's home too.
Then in Avengers: Secret Invasion, we start with Agent Ross working with a Dora Milaje crossing paths with Maria Hill and Sam Wilson. Both groups are surveilling a deal where an agent of the Power Broker is trying to sell Talokanil DNA to an agent of the Ten Rings. A shootout happens, and the Ten Rings agent escapes, but Ross gets hit and dies . . . revealing him to be a Skrull. Cue the paranoia.
The plot is basically that Fury and Talos realize that Giah has begun bringing in more Skrulls to infiltrate powerful organizations, and so they decide to assemble a team of super-powered individuals -- whom they assume can't be simulated by Skrulls -- to figure out what Giah's up to and bring her in, hopefully alive.
But secretly, Giah is trying to get samples of super-powers so her people can be strong enough to defend themselves once their infiltration is discovered.
This set-up lets you bring in Captain Marvel (who despite being incredibly powerful can't solve this issue with energy blasts), Captain America (whom Fury doesn't trust because he has no super powers), Shang-Chi (who gets involved because his sister who is head of the Ten Rings asks him to), and Shuri (who is obligated because of her people's alliance with Talokan).
(Fury reaches out to Dr. Strange to help them detect Skrulls by, y'know, punching their souls out of their bodies, but he's missing.)
Maybe Valentina sends Yelena to get involved, because Ross was Valentina's ex (and, y'know, it'd be nice to keep the world from being overrun by aliens).
And heck, bring in Sprite from the Eternals, since she was left on earth and is sort of a shapeshifter. Could be a fun dynamic, though I'm not sure how to get her embroiled in the plot.
Now, what exactly to do with the cast, eh, I won't overthink it since it's never gonna happen. But yeah, have a fun spy thriller. Have some people be revealed as Skrulls. The showdown can be at the facility where they're keeping all their prisoners so they can continue to simulate their minds. Have it be a twist that actually Giah's Skrulls can have super powers.
