MCU: Why Natalie Portman Is Becoming Female Thor
4. The Missing Mjolnir Issue
Despite the flirtations with introducing Valkyrie as a new love interest to Thor in the MCU, the gag rug-pull showing her not to be interested was a very good one and it paves the way for the Love in the title to be as much about Thor and Jane as it is about Valkyrie's quest for a queen. But this won't be another case of Jane coming in purely as a love interest and Thor taking all of the action. She will be worthy and she will wield Mjolnir, probably in service of the new Asgard and Valkyrie as the new king.
You could ask why Valkyrie wouldn't inherit Thor's position as Asgard's protector as well as the throne, but Odin still kept his son around despite being the most powerful entity in the universe. He didn't wield Mjolnir because he didn't need to (his staff Gungnir is more powerful) but also because ruling the combined kingdoms was his role rather than being a warrior, as Thor was. There's also an issue with assuming that Valkyrie would be worthy, since there's been no confirmation of that yet.
More crucially, Mjolnir no longer even exists in the main timeline. It was destroyed by Hela in Thor: Ragnarok and the "replacement" hammer retrieved by Thor in Endgame was returned to the past (the Dark World time). Valkyrie couldn't wield something that no longer exists.
So, a couple of things have to happen before Jane Foster can wield the hammer. First, Mjolnir needs to be back in play. We can either see that happen through someone time-travelling again, but the likelihood of them doing that twice in such close proximity is just not logical. It would undo all of Endgame's ending and make it seem like anyone can just leap about in time fixing things as they like. When you introduce things like that, you remove all threat entirely.
More likely is the idea of the Multiverse being the key to Mjolnir reappearing somehow.
In Doctor Strange In The Multiverse Of Madness, we're clearly going to see more alternate universes, which will probably be responsible for the reappearance of Vision for the WandaVision Disney+ show. What if that is also the key to Mjolnir turning up again? It's not like it can simply be a matter of a new hammer being forged - that's not how Asgardian artefacts work. It has to be the same hammer. And if it's from somewhere or somewhen else, then it is the same.