Marvel Studios had the opportunity to properly reintroduce Jane Foster in Avengers: Endgame owing to the film’s time travel subplot, but they instead decided to give it to Tony Stark.
In terms of understanding the laws of astrophysics and interdimensional travel, you can’t find a better candidate than Jane. In fact, when you consider the first two phases of the MCU, there’s no scientist with firsthand experience with the Infinity Stones other than Thor’s girlfriend and her associate, Professor Erik Selvig. But the two essentially disappear from the overarching story after The Dark World.
Then again, the franchise has made it clear that Jane’s still around. Age of Ultron even explains that she’s become an acclaimed scientist in the field of astrophysics and the screenplay for Endgame notes that the physicist was among those who perished after Thanos’ snap. When you think about it, though, the producers could have actually brought her character back in full force, rather than just for a brief cameo in Asgard, to help Earth’s Mightiest Heroes figure out the dilemma of time travel.
I mean, who better to tackle the mysteries of the Quantum Realm than the MCU character who’s dedicated her entire life to the pursuit of the impossible?
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At the end of the day, though, the writers decided to give this subplot to Iron Man as well, even though his expertise doesn’t really have anything to do with quantum physics. Still, it’s an understandable approach. After all, Avengers: Endgame was always supposed to be the ultimate swansong for the hero that started it all and bringing back Jane to figure out the time tunnel would’ve just shifted focus from the main characters.
Besides, we know that she’ll return as a main player in the upcoming Thor: Love and Thunder, becoming the Goddess of Thunder and wielding Mjolnir, no less. So, in a sense, even if there was a narrative opportunity there, her absence probably worked out for the best.