The Mjolnir will be passed on to Natalie Portman's Jane Foster to become The Mighty Thor in Thor: Love and Thunder. However, a new theory pointed out that Thor: Love and Thunder could be setting up to introduce Beta Ray Bill.
Beta Ray Bill is an alien warrior who has similar costume and power of Thor. He was one of the character on comics who is worthy of wieding the Mjolnir and later got his own weapon, a half-hammer and half axe called Stormbreaker which was already been introduced in MCU.
According to Lazar Shell from Reddit, Thor: Love and Thunder will address Thor's love story which hasn't been resolve and Lady Sif and Beta Ray Bill romantic relationship.
"Jane - She's already confirmed, she's also going to take Mjolnir. She's the first character that's going to tie the themes of Love and Thunder together. Sif - Thor's second love, the love he's going to be torn over. Do you choose the mortal or the goddess, do you choose the one you are destined to be with or the one you actually love. And since after Endgame Thor's journey is making his own journey, not following destiny, I think he will choose Jane. We also saw him still hung up on her.
Beta Ray Bill - The second character that will tie the themes of Love and Thunder together. The thunder bit is obvious, he'll take Stormbreaker or get a new hammer that's going to give him Thor's powers. He'll also be tied into Love since in the comics him and Sif have a romantic relationship, Beta Ray Bill was the first mortal that made Sif understand why Thor was so in love with Jane.
Enchantress - I think, more than anyone else listed here, Amora is the most obvious choice for an antagonist. I don't know how you can have a Thor movie about Love and not have the character whose sole purpose and power is just that. She's also one of the few villains not used in any aspect in the movies (Hela was a combination of a lot of villains: Hela, Mangog, Gorr, Cul and Angela)."
The main plot of the movie will be about Jane becoming The Mighty Thor. Natalie Port man previously told Variety that she likes about the cancer storyline. In the comics, Jane Foster has cancer but she gets stronger every time she uses Thor’s hammer and transforms into The Mighty Thor. Portman said she likes the idea of her character facing two battles in her journey to becoming The Mighty Thor but she did not confirm if it will be the plot in “Thor: Love and Thunder.”