However, thanks to Endgame's five-year time jump, Cassie's return required an older actress, Emma Fuhrmann, whose brief moments on-screen packed an emotional punch.
This is why fans were surprised when Marvel Studios confirmed that Kathryn Newton - not Emma Fuhrmann - would play Cassie Lang moving forward and in Ant-Man 3.
While the reason for the switch from Fortson to Fuhrmann was obvious, Newton's casting for the threequel was not, leaving audiences confused about the significant change.
Why Was Cassie Lang Recast In Ant-Man and the Wasp: Quantumania
While Ant-Man and the Wasp: Quantumania already has a crowded call sheet, Kathyrn Newton is expected to have a large role in the film.
According to Ant-Man 3 director Peyton Reed, time is the central focus of the film, and much of that has to do with Newton's Cassie becoming an adult and Scott Lang wrestling with how much time he's lost with her.
Meanwhile, Newton has claimed that Cassie will be "making a lot of mistakes" as she learns how to become a hero herself.
Given her importance in Ant-Man 3, her own character's arc, and the fact she would be sharing the screen with a lengthy list of established talent, it appears that the studio wanted a more seasoned actress to shoulder the role.
What's unfortunate about Marvel recasting Cassie Lang is that Emma Fuhrmann apparently only found out when the official announcement was made.
Following the reveal, the actress posted on Twitter that she was sad "to hear the news Thursday" and expressed hopes that there would be "something else for me in the future of the MCU."
How Ant-Man 3 Positions Cassie Lang for a Leading Role in Future MCU Projects
While only time will tell how fans respond to Kathyrn Newton's portrayal of Cassie Lang, it's important to remember that Marvel Studios casts for the long haul.
Quantumania isn't the actress' ultimate test but rather, it's only her first chapter.
It's also worth noting that Ant-Man 3 is likely to position the character for a leading role in future films or Disney+ series.
Ant-Man and the Wasp: Quantumania arrives in theaters on February 17.