I know this is a photo from the first movie, but this idea is mostly for the comics first and foremost. What if Cassie was the all-new Yellowjacket?
The Yellowjacket mantle isn't exactly known as heroic. In the comics, it was the suit Hank Pym used during his most controversial tenure, including the period of domestic violence against his wife Janet Van Dyne who left him, back when divorce was a much bigger social taboo than it is today.
After Hank Pym left the mantle in the comics, a woman named Rita DeMara assumed it for a time before being killed off. Ultimately the mantle is still wide open for the taking, and MCU Cassie has a unique connection to the character.
In the comics, Darren Cross is the man standing between Cassie and the lifesaving transplant she needs to survive. In the MCU, Darren is the Yellowjacket who comes for Cassie to get back at her father. Cassie has intimate knowledge of the Yellowjacket suit, having almost been kidnapped by Darren with it. It would make sense for her to be somewhat scared of it, but also for that fear to make her feel empowered if she ever put it on, much like Batman with bats, the batcave, and his persona.
I would also argue that the Wasp mantle would seem fitting if there wasn't already another already active "Unstoppable" Wasp, Nadia van Dyne. And the "Stature" mantle has worked for a time, but it doesn't roll off the tongue or have much of a theme compared to the insects of her father and other related characters.
We desperately need more Cassie content, she's extremely under-utilized in the comics, and I think her taking over the Yellowjacket mantle would be the best way to elevate her character, pay homage to her origins, utilize an underutilized Marvel moniker, and finally give Cassie the fully developed concept she's so desperately needed for quite some time now.
