Shang-Chi is rumored to involve Iron Man 2 and 3 and to take place during "The Snap," which would make the movie another prequel MCU flick.
The Snap took place for five years between The Avengers: Infinity War and The Avengers: Endgame, which saw the Great Titan Thanos wipe out half of all life in the universe, but Tony Stark brought everything back at the end of Endgame.
According to a leak posted to 4Chan, Shang-Chi, "as most of the leaks have already detailed," features "a spy movie/tournament arc set during The Snap."
Shang-Chi and the Legend of the Ten Rings has a May 7, 2021 release directed by Destin Daniel Cretton and stars Simu Liu as Shang-Chi, Tony Chiu-Wai Leung as The Mandarin, and Awkwafina in an unknown role.
Shang-Chi grew up working for the Mandarin after having been essentially adopted by him. Awkwafina plays Fah Lo Suee, who is basically his sister. The three of them actually have a genuinely loving and fun family dynamic.
Shang-Chi thinks they are the good guys, keeping the world in order while Mandarin seeks the Ten Rings. He's been helping Mandarin behind the scenes of the MCU since he was a kid. Mickey Rourke and Ben Kingsley from Iron Man 2 and Iron Man 3, respectively, have cameos related to these flashbacks.
Shang-Chi was tasked with killing Trevor Slattery (Ben Kingsley) but couldn't do it and let him go. Trevor ended up picked up by authorities in London, which clued in M-I6 and Clive Reston.
Shang-Chi is fighting in the tournament at the behest of Mandarin to win the 10 Rings. The tournament is being held by a mysterious shadow figure.
The governments/organizations of the world are desperate to get their hands on this alien tech because The Snap was so devastating, they need to be prepared.
Lots of cameos are featured in the tournament, paying off other shows/movies and setting new stuff up and running.
Baron Mordo, Batroc the Leaper, a Black Widow, Whiz Kid, White Claw, Omega Red, Spiral, and T-Ray all appear, along with an Asgardian named Dane, and a vampire named Avok.
Seems like mutants are already in existence by this film, possibly introduced in something earlier.
T-Ray gets a big showcase so he may be a major future threat in something (Deadpool 3?)
There's a really funny reoccurring bit about Clive Reston discovering more and more new kinds of powered people like Spiral and Avok.
Baron Mordo seems to be written out of Doctor Strange 2 in this movie.
Mandarin gets the 10 Rings after Shang-Chi wins the tournament. He inadvertently summons Fing Fang Foom. Shang Chi, Fah Lo Suee, Mandarin and Clive Reston defeat FFF together. The. Fah Lo Suee and Shang Chi defeat Mandarin.
The film ends with an epilogue of Shang-Chi in the present day.