The project is still in early stages, with no cast announced, but the Thunderbolts concept is fairly straightforward - a bunch of bad guys work as 'heroes' for the government (might sound a little familiar to fans of Suicide Squad of the Dirty Dozen). We are not sure which characters will make up the team, though Deadline has some sources that say Marvel has already contacted a few actors already in the MCU for shooting next summer.
However, since there are only so many living bad guys in the MCU, we can make some educated guesses as to who might be on this mysterious team. The most obivious choices would be Baron Zemo (who was in the original team and has recently appeared on the Disney+ Falcon and Winter Soldier) and Yelena Belova (who was recently on Disney+ Hawkeye). Because Abomination is going to be in the She-Hulk show, it may make sense to include him as well. The list of Thunderbolts members over the decades is so far-ranging, it could also include recruiting some of the Netflix-era Defenders - afterall, Matt Murdock has already re-appeared in the MCU.
The Thunderbolts originally appeared in Incredible Hulk #449 in the late '90s, and were created by Kurt Busiek and Bark Bagley. The original team included Baron Zemo, Beetle, Fixer, Moonstone, and Screaming Mimi. Out of these characters, only Baron Zemo has appeared in the MCU, but Marvel Studios is no stranger to introducing new characters in team movies (Guardians of the Galaxy, The Eternals). Currently, in Marvel Comics, the newest iteration of the Thunderbolts have been enlisted as New York City's official superhero team, and they currently report to Luke Cage, mayor of New York City.
To read more about what's going on with the Thunderbolts in Marvel Comics right now, check out this article on Luke Cage being the mayor of NYC.