MCU: Why Natalie Portman Is Becoming Female Thor
2. Jane, The Aether And Death
Think back to Thor: The Dark World and Jane's "infection" with the Aether. It's fairly firmly established that Jane is in danger of dying for as long as the Aether is absorbed within her. That drives the whole narrative for Thor after she's infected: it's not just a matter of saving the Nine Realms, he's more personally invested because he's saving Foster.
Ultimately, she's saved and the Aether is removed from her, but something like that, which would be deadly for prolonged periods of exposure, would surely cause damage while it was in contact with her DNA? What if the Aether infection caused Jane irreversible genetic damage, which is killing her when she reappears in Thor: Love & Thunder?
What if Jane has reached the end of her options and heads to Asgard - which she'd surely be aware was on Earth now - to try and find a magical solution to her dying? That would be the perfect set up for Valkyrie to grant her the powers of Thor, having somehow retrieved Mjolnir from another dimension. It's not like that contrivance would be any more ridiculous than suggesting that Jane and the hammer have a telepathic link.
And when you think about it, having Valkyrie make Jane Thor and then them both keeping the secret from Thor would be somewhat fitting, giving those circumstances. It adds an even more tragic note to the whole dynamic.
The last thing to explore is why Thor wouldn't be Thor anymore and why he'd be replaced.