Thor's cancer will eventually kill her.
Her terminal diagnosis is a Foregone Conclusion of sorts, suggesting she'll only be Thor for a finite period of time, unless she's cured by some divine intervention before it can happen. But wouldn't that be kind of a disappointing cop-out, narratively speaking? And Thor (Odinson) has a new movie coming out in 2017 anyway — they're going to want some corporate synergy going for promotional purposes.- That she wont be Thor permanently is also a foregone conclusion thanks to several series (Aaron's God Of Thunder, Ewing's Ultron Forever etc.) showing Asgard's future on different points in time and all have Odinson as Thor again.
- The argument could be made however, that these are alternate future— Marvel is notorious for using that as a way to get out of corners they've pointed themselves into— and that Femme Thor will continue to wield the Hammer until she either stops drawing money, or people just stop calling her Thor and using her real name.
- They didn't go with synergy for the Ragnarok movie, but she did eventually die in 2018's Mighty Thor #705.
Thor will become the new Ultimate Thor.
There is one possible reason why that Mjölnir got thrown into the new universe... It even has a gender neutral (as in one using singular they) inscription.- Jossed, Volstagg took that power instead.
Odinson will use Ultimate Thor's hammer upon his return.
The hammer is a much heavier weapon, and is a hammer-axe hybrid. Odinson used an axe before he lifted Mjolnir, and thus it wouldn't be a problem for him. Jane's fighting style is also heavily reliant on Mjolnier being quick and agile, which it is unlikely Ultimate Mjolnir is due to its size. With Odinson missing, it's very possible he travelled to Old Asgard and found the hammer.- Jossed. While he does find the hammer, he refuses it, and instead determines to reclaim the original.
Odin's increasing madness is actually the doing of Freyja.
Freyja's power struggle with Odin is a key story element, and we know from Loki: Agent of Asgard that she's not above hurting her family to get what she wants. Meanwhile, Odin's behavior is clearly and blatantly abnormal, just from the way the character react-while he's got a temper and often fails to apply the wisdom he's so famed for, now he regularly goes too far to the point that even the Asgardians are shocked. This comes to a head when Odin has Freyja scheduled for execution...prompting a popular revolt in her favor. But we know that Freyja never intended to die that day, apparently having plotted with Loki, a plot that was on-track enough that she chastised him that him busting in with Thor to save her "wasn't the plan." So what was the plan? In all likelihood, it would have involved using Loki's trickery to either fake her death or avert it in some other fashion so she could lead this burgeoning revolt, having determined who she could trust, and focus on the problem of the Dark Council (which Odin, conveniently and uncharacteristically, has completely ignored), before ultimately securing her power by right of Odin's obvious madness. Ultimately Odin's madness benefits Freyja's bid for sole power over Asgard by making her look like the sane one who can deal with these problems. Loki then derails this plan with his poisoning, and it's notable that Odin, who was ready to kill her the previous issue, immediately seems to snap out of his madness and retreats with her to his quarters. Jane's Thor form being an illusion is done by Mjolnir as a way to show Jane humility in the face of her hubris since gaining control of it.
This makes a lot of sense. Thor's hammer is typically shown to have distinct transformative capabilities that it enacts on its wielders. Beta Ray Bill and Red Norville aside, we've seen that the Hammer DOES physically change the bodies of those who wield it (Eric Masterson and Dargo are very blatant examples of this.) So the question becomes, why the hell is her form considered an illusion, rather than a completely different body?! The answer is simple. Jane is still determined to use Earth medicine to deal with her cancer, instead of accepting aid that would help her survive because she thinks accepting help from other worlds is some kind of a cop out. This despite the fact that she sits on a congress of various worlds in Asgardia and assumes that Earth needs a say in such things. She has the ability to use medicine that may well be superior— or at least more effective— than what she's getting on Earth. This is a form of hubris that Mjolnir is annoyed by, so even while it gives her power as female Thor, it doesn't give her a true Thor equivalent form, but instead just gives her the power and health of a Thor, but not the actual body, showing that her worthiness may not be as real as she thinks by making her Thor form an illusion rather than the actual form it appears as. It's trying to teach her a lesson in humility, but she refuses to see it.