Marvel's slightly different from the source material, it's likely that they could be making a villainous turn in the TV show.
Some Skrulls have already been teased amongst humanity, with Maria Hill and Nick Fury once being replaced by the shape-shifting aliens. With so many well-known characters in the MCU, there are a few ing players who could certainly be useful in the larger invasion plan — people who may have already have shown signs of a personality shift...
Phil Coulson
Agents Of S.H.I.E.L.D. For the most part, the team didn't know that the agent had actually survived his run-in with Loki. However, he has now been replaced by a robotic duplicate.
Phil Coulson could very well appear in the MCU again, with his status certainly confusing Earth's Mightiest Heroes. Nick Fury would have knowledge of Coulson's LMD legacy, but it would be easy for a Skrull imposter to take this android's place. The story of Coulson's many deaths has enough gaps to confuse those around the Skrull into believing that they are now the Life Model Decoy.
Sharon Carter
Sharon Carter has had a notable personality change in her appearance in Falcon And The Winter Soldier. When audiences last saw her she was on the run from the authorities, after helping Captain America get his gear back. Time has not been kind to her in the years since.
She is now going by the name of the Power Broker and has seemingly set up a criminal empire. This could be part of the character's evolution, but these personality shifts may also be as a result of a Skrull replacing her during the years of the Blip.
Happy Hogan
Happy Hogan is definitely a useful character for the Skrull Empire to take control of. He is in the middle of the superhero community, has control over a lot of old Stark assets, and is close to the real-world identities of heroes like Spider-Man. Happy is a logical person to replace.
It's unclear what Hogan was actually up to during the Blip years. It's very possible he could have been swapped out for a Skrull throughout this time. The only person who could detect it would have been Stark, and with Tony gone, Hogan is unstoppable. His next appearance in No Way Home should provide some answers.
Tyler Hayward
Tyler Hayward was one of the main antagonists of WandaVision. He holds a high position in S.W.O.R.D., which has already been proven to contain some Skrull agents. With characters like Monica not around to stop his rise, he accumulated power without anyone finding out his real identity.
His actions in WandaVision were certainly a little questionable. He was portrayed as villainous without a real goal in mind. Perhaps this was a Skrull testing the limits of one of Earth's most powerful heroes and hoping to bring White Vision under his control for the invasion.
Valentina Allegra De Fontaine
One of the things about Skrulls that can be learned from the comics is that they are very good at finding power vacuums and filling them with fake identities. Valentina Allegra De Fontaine has really come out of nowhere and could hold a huge role in the future.
This character could be a Skrull, plotting to create a Dark Avengers team that will sow chaos and distrust amongst the superhero community. This would be a useful strategy to destabilize Earth's defenses. Not much is really known about Valentina's background, making her a perfect candidate.
Nick Fury
Nick Fury has already been replaced by a Skrull once on Earth. Who's to say that it couldn't happen again? Fury is currently on a Skrull ship, with many people assuming that this is linked to the S.W.O.R.D. space project. Monica Rambeau may even be on her way to see him.
Yet, something doesn't seem right about this arrangement. Perhaps the Skrulls are keeping Fury away from Earth and unknowingly replacing him to run things. No one but Maria Hill knows that he's with the Skrulls and since she has disappeared, it's possible that these allies are taking advantage of the situation.
Thaddeus Ross
Thaddeus "Thunderbolt" Ross was obsessed with taking down the Hulk when he first appeared in the MCU. But since then he's supposedly completely shifted his life, becoming a governmental official and overseeing the Sokovia Accords.
Did Ross really have a heart attack and a change of heart, or did a Skrull take over and begin to put their own plans into action? It's a pretty convenient position for Thunderbolt to be in since he can have the identity of all the powered individuals on the planet at his fingertips.
Pepper Potts
Pepper Potts and Tony Stark might have been the perfect couple. Potts' personality was set out very clearly, though, with the character not really approving of how dangerous Iron Man's life really was. At one time he even gave up the suits for the love of his life.
Her opinions have somehow shifted so much that she's willing to jump into battle as Rescue. As a hero in her own right and now the owner of Stark's companies, it wouldn't be completely out of the question for Potts to have been replaced by a Skrull at some stage during the Blip.
Clint Barton
Clint Barton's sudden personality change into a more vicious Ronin could be a sign that he may have been replaced by a Skrull. He went off-grid for so many years that anything could have happened to the former Hawkeye. Out of all the main Avengers, the subtle shifts in Clint may mean he's the most likely to be a Skrull.
Compared to other characters, though, Barton's replacement would undoubtedly be a sleeper agent who is struggling with their identity. The comics see some of the first Skrulls found in Japan, with Elektra making the Avengers aware of the incoming threat. With links to Japan and the character of Echo, a similar narrative could be playing out in the MCU.
Sif
Lady Sif has been missing for so long that she wasn't even around for the destruction of her home, Asgard. There have been teases that she will return down the line, but there has to be a good explanation as to why she has been MIA throughout the last few years of the MCU.
Her adventures in space might have seen her captured and replaced with a Skrull agent. Considering her standing in Asgardian society she would be a useful ally to have on board. She could destabilize what's left of New Asgard before the incoming invasion.