Marvel's officially confirmed Thunderbolts were originally a team of supervillains masquerading as heroes following the presumed death of the Avengers. A later version of the story saw the team tracking down uned villains for the government following the events of Civil War and the Superhuman Registration Act.
The MCU has been seeding the Thunderbolts for years with major characters like Baron Helmut Zemo (Daniel Brühl), introduced in The Falcon and the Winter Soldier's John Walker (Wyatt Russell) and Black Widow's Yelena Belova (Florence Pugh). No doubt, Contessa de Fontaine will find other heroes and villains to the group she represents, which is funded by a secret but powerful organization.
Thunderbolts is also an opportunity for the MCU to bring back characters from the early Phase 1 films and give them a new purpose. This is something the Marvel Cinematic Universe has been very successful with, as is evident from the return of Obediah Stane (Jeff Bridges) and Ivan Vanko/Whiplash (Mickey Rourke) are deceased, here are some of the familiar faces who have been absent from the MCU for many years and who could make a comeback in the Thunderbolts movie.
Justin Hammer
Justin Hammer (Sam Rockwell) has been primed for a comeback ever since his arrest at the end of Justin Hammer's Iron Man 2 arrest and public humiliation were nearly 15 years ago in the MCU's timeline. It's quite possible that in the post-Blip world, Hammer is a free man and back in business. Providing funding and weapons for the Thunderbolts could be his ticket back to the big time now that his arch-foe Tony Stark is deceased.
Secretary Ross/Red Hulk
General Thaddeus Ross (William Hurt), whose involvement goes all the way back to the creation of Captain America's super soldier serum, has been a thorn in the side of the Avengers ever since he became Secretary of State and implemented the Sokovia Accords. Ross was seen in hunting the fugitive Natasha Romanoff (Scarlett Johansson). And of course, while the film is hardly talked about, Phase 1's The Incredible Hulk introduced Ross' hatred for his almost-son-in-law Bruce Banner.
As Secretary of State, it would make sense that Ross would want a stern hand in the creation and oversight of the Thunderbolts as a new super-team to supplant the Avengers. In fact, just as Nick Fury (Samuel L. Jackson) named the Avengers after Carol Danvers' (Brie Larson) US Air Force callsign, Ross' classic nickname "Thunderbolt" could be the basis of the newest team name in the MCU. There's also the possibility that General Ross follows his comic book counterpart in becoming the MCU's Red Hulk, something that has been long-discussed among fans.
Samuel Sterns/The Leader
The lone appearance of Dr. Samuel Sterns (Tim Blake Nelson) in the MCU thus far was in 2008's The Incredible Hulk. A professor at Grayburn College and a noted cellular biologist, Sterns was in secret with Bruce Banner (Edward Norton) about his gamma mutation. By the end of The Incredible Hulk, Sterns was accidentally exposed to Banner's gamma-irradiated blood, which caused his own mutations. In Marvel Comics, Samuel Sterns is a diabolical mastermind known as the Leader and an arch-enemy of the Hulk. The long-forgotten character could finally reappear in MCU Phase 4 as the Leader, possibly as one of the behind-the-scenes forces influencing the Thunderbolts, if not as an actual team member.
Emil Blonsky/The Abomination
Shang-Chi and the Legend of the Ten Rings. Blonsky was a British soldier who received the Super Soldier Serum to stop the Hulk. He then forced the aforementioned Samuel Sterns to infuse him with Bruce Banner's gamma-irradiated blood, transforming him into the Abomination. Hulk ultimately won the battle between the two monsters, and Blonsky was imprisoned for many years.
When he returned years later in Shang Chi, Blonsky was surprisingly a cage fighter in Macau, China who also happened to be a friend of Wong (Benedict Wong). His character is expected to return in the Disney+ series She-Hulk, but it's worth noting that the Abomination was originally earmarked for the Avengers Initiative prior to the Hulk being created. That could be all the motivation he needs to up with a team of super-villains should the offer arise. By ing up as the Thunderbolt's gamma-powered counter to Hulk, the Abomination can finally fulfill the role he was originally meant for, and given his connection to General "Thunderbolt" Ross, the idea isn't too far-fetched.
Christine Everhart
Christine Everhart (Leslie Bibb) was memorable as an early fling of Tony Stark's in alt-universe version of Christine Everhart. Considering her predilection for working with questionable people like Justin Hammer, Everhart could return in live-action in Thunderbolts.
William Ginter Riva
William Ginter Riva (Peter Billingsley) was a Stark Industries scientist who failed to duplicate Tony Stark's design of the Arc Reactor in Iron Man. Yet Riva held a grudge against Stark instead of Obediah Stane, and the scientist returned years later in Spider-Man: Far From Home. This time, Riva worked with Quentin Beck AKA Mysterio (Jake Gyllenhaal) to take control of Stark's EDITH technology. Riva remains at large after the events of Far From Home and he played a crucial role in doctoring the footage of Spider-Man (Tom Holland) fighting Mysterio to make it look like the wall-crawler killed the illusionist. It's also thanks to Riva that the footage came into the possession of J. Jonah Jameson (J.K. Simmons), who publicly exposed Spider-Man's identity as Peter Parker. It's not a stretch to imagine Riva could reemerge to supply his scientific talents in service of the Thunderbolts.
