According to a podcast interview by Josh Horowitz where he talked to directors Joe and Anthony Russo, Katherine actually played the older version of Tony Stark's (Robert Downey Jr.) daughter Morgan (Lexi Rabe) in a deleted Endgame scene.
Here's what was supposed to happen: Iron Man was going to be transported to an ~ethereal plane~ after stealing the Infinity Stones and attaching them to his own gauntlet to snap away Thanos and his army. Once he got there, he would meet a future version of Morgan, whom he's about to leave behind. "The intention was that his future daughter forgave him and gave him peace to go," Joe Russo said in the interview.
So what happened?
"We showed it to a test audience and it was really confusing for them," Joe said. "What we realized about it was we didn't feel an emotional association with the adult version of his daughter. So it wasn't resonating with us on an emotional level."
Aww, it probably isn't the right time for Katherine to be part of the MCU yet.
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