The fourth installment of Marvel Studios' Thor series, Thor: Love and Thunder, will start filming early 2021, which is very exciting, especially with someone else taking on the Thor mantle. At Marvel's 2019 Comic-Con International panel it was announced that Natalie Portman's Jane Foster would return and become the Mighty Thor. Jane previously appeared in the Thor movies as his love interest, but this movie will highlight and expand on her character as she becomes the Mjolner-wielding Goddess of Thunder. Details about exactly how Jane becomes Thor are not yet known; however, fans have a plethora of Marvel comics and MCU films to speculate from.
In the comics, Jane endures a harrowing journey as the new Thor. First, the original Thor becomes unable to lift Mjolnir and loses his status as God of Thunder, leaving the title up for grabs for anyone worthy. The hammer summons the next worthy person, which happens to be Jane, so she takes on all of the responsibilities of Thor. If the hammer, which is still in existence in other timelines, does call for a new master in the MCU, Portman's Jane could be the one summoned, like in the comics.
Being Thor is exciting, but like all heroism, it comes at a cost. While she is Thor, Jane is also battling cancer, but she doesn't let this stop her from doing what is right as the Goddess of Thunder. However, in doing so, the chemotherapy is being undone by Mjolnir, so being Thor is killing her.
She knows that if she wants to live, she must stop being Thor; nevertheless, she picks up the hammer one last time to save everyone at the expense of her life. Whether or not the film includes Jane's cancer, it could see her suffering physically from taking on the role of a god given that, as the comics establish, all magic comes with a cost.
Along with looking at her time as the Mighty Thor in the comics, the MCU can look back at the previous films to set up this version of the Goddess of Thunder. While the movie isn't in production yet, fans are starting to theorize how Jane will become Thor, including Redditor u/TheMediocreCritic's theory that Jane will be from an alternate timeline created by the Loki who escaped in 2012, as seen in Avengers: Endgame.
This theory specifically proposes that in this new timeline, Thor is dead, so a new Thor is needed. In his absence, Jane proves herself worthy of the title and becomes the Goddess of Thunder. Even if he isn't dead in the alternate timeline, he could be unworthy of the hammer, which would tie back to the comic origin of Jane's Mighty Thor.
The comic and film lore Marvel as a whole has generated over the years in regards to Thor Odinson and Jane Foster is enough to set up the introduction of the Goddess of Thunder in the MCU. Fans theories are popping up about how Jane becoming Thor is possible; however, fans don't need to look far to learn how The Mighty Thor comes to be, starting with Jason Aaron, Russell Dauterman and Matthew Wilson's Thor #1 (2014), which first introduced the Goddess of Thunder.