Thor fights. No matter who holds the hammer, be they he, she, or it (shoutout to Throg, the worthy frog), they fight. Frost Giants, trolls, Dark Elves, and beings powerful enough to destroy entire universes. But Jane Foster isn't just embattled while she wields the hammer.
Every time Jane becomes Thor, the powerful magic that flows through her purges all toxins from her body, including the radiation and chemotherapy drugs needed to beat back her cancer. While she's Thor, everything is okay. But as soon as she changes back, the lack of medicine allows the cancer to spread more and more. She has pledged to never give up on being Thor, but it's only a matter of time until she must be cured, or perish.
But What About the Odinson?
While Jane has been adventuring as Thor, the Odinson has been doing some soul searching. He has popped up in Thor on a few occasions, but he continues to wield Jarnbjorn, his enchanted axe. Now, he, too, has his own series. The Odinson is headlining Unworthy Thor. As the cover shows, he is currently seeking out what used to be Ultimate Thor's Mjolnir.
In Unworthy Thor #2, he was shown to possibly be worthy of it, suggesting that he may have claim to his own, as well. Unfortunately, it's in the hands of the Collector, so he certainly won't have an easy time with it.
The Odinson has gone through a lot since he lost Mjolnir, and the introduction of this series at this point is not arbitrary. Both Thor and the Odinson are reaching tipping points. Thor's sickness and Odinson's unworthiness cannot continue indefinitely. It is all but certain that their stories will intersect again at their respective climaxes. But why exactly could this mean Jane will die?
Marvel's History With Cancer
A previous Marvel event gives us some clues. In The Death of Captain Marvel, Mar-Vell lost his life to cancer after many tried and failed to cure him. This was Marvel's last real attempt at dealing with a disease that can strike anyone at any time. They aimed to show that cancer was a threat to everyone, and that we needed to raise awareness and money to cure this deadly disease.
Currently, breast cancer, the form Jane is afflicted with, is one of the most common types of cancer. If Marvel is hoping to raise awareness and gain attention, Jane's loss would certainly do so. While her survival could be an inspiring story, it won't gain the attention her death could.
So How Might This Go Down?
It's my belief that as Jane dies, she will pass the hammer back to the Odinson after a climatic battle in which the two will fight side by side. It will fully establish the worthiness of both characters, but when Jane changes back, it will have been her final battle. Only time will tell whether this prediction is accurate, but it's safe to say that the future of Mjolnir is far from certain.