Following his most well-received movie ever, both critically and commercially, everyone is hot on Thor right now. Thanks to the vision of Taika Waititi and the comedic timing of Chris Hemsworth, Thor: Ragnarok took the God of Thunder into uncharted territory, making him one of the funniest but no less powerful characters in the MCU. Thor is next set to appear in the universe-redefining Avengers: Infinity War and Avengers 4, but after that, only Marvel Studios knows what's going to happen to him. Avengers 4 could very well be the last we see of Thor, but if that were to happen then Marvel already has a perfect character ready to step up and take his place: Valkyrie.
The future of the Marvel Cinematic Universe is a big question mark right now. Characters could equally be killed, retired, or recast. Should Chris Hemsworth decide not to return to star as Thor, that leaves the door wide open for Valkyrie to become the new face of his franchise. Introduced in this month's Thor: Ragnarok, Valkryie is one of the best new characters Marvel has ever introduced. She stands as an equal with Thor and Hulk throughout Ragnarok, and from the moment she drunkenly falls out of her spaceship, she's already won us all over. With an excellent performance from Tessa Thompson, she's got the charisma, swagger, and the physical action prowess needed to support her own franchise.
As to how Valkyrie would take over, we can only write our own fan fictions, but my ideal version would be Valkyrie acquiring a re-forged Mjolnir at the end of Avengers 4. This hammer would give her all the powers of Thor like flight and control of lightning. This is as opposed to her simply sticking with her own formidable Asgardian powers, mostly because there's more meat to the idea of worthiness and what that means to her. However, hammer or no hammer, she could still totally own the movie, as Ragnarok proved.
As comic book fans well know, there is a huge amount of precedence for someone taking over the role of Thor. The biggest and most relevant one to this argument, though, is the female Thor that was introduced in the comics. After Thor became unable to lift his hammer, his longtime on-again-off-again girlfriend Jane Foster found herself worthy to wield Mjolnir, gaining all the powers of Thor and then some. Though Jane struggles with cancer (which is only made worse by her transformations into Thor), she proves that she is every bit as worthy as her male counterpart, unlocking powers in Mjolnir that Thor never could. Her comic book, "The Mighty Thor" written by Jason Aaron, has been one of the most consistent Marvel books on the stand, so there's a lot to mine for a live-action adaptation.
Jane Foster is pretty much a non-factor in the MCU, having never had a particularly strong presence. (That's not your fault, Natalie Portman.) She was further shuffled away with one line of quick dialogue in Thor: Ragnarok, so it'd be very shocking to see Marvel just hand her the keys to her own franchise. However, Valkyrie has only appeared in one movie thus far, and people are already asking when we're going to see her again. At the end of the day, she's just a more memorable character and a more logical choice to lead the franchise.
When we're introduced to Valkyrie, she's basically hit rock bottom and is living in denial. After watching her fellow sisters-in-arms die by the hands of Hela (Cate Blanchett), Valkyrie ran away from her problems, becoming a bounty hunter on Sakaar, about as far away from Asgard she can get. She pretty much sentences those she captures to death in the Grandmaster's Contest of Champions and drinks away her memories. She eventually stops running from her problems, helping Thor to save Asgard, but that doesn't mean she's gotten over her trauma or guilt. It's a fascinating character arc that would be amplified by putting a hammer in her hand.
Thor and Valkyrie have plenty in common, but it's their difference that will ultimately keep the Thor series exciting in Phase 4. Whereas Thor makes an effort to look cool in Ragnarok, Valkyrie pulls it off effortlessly. She's a scrappy, roughish figure that counteracts Thor's nobler attitude. She can slay monsters with the best of them and then outdrink the rest. The longer the MCU lasts, the more it has to do something new and different. Valkyrie leading Thor helps with that.
It also helps that Tessa Thompson is an extremely talented actress. It looked like she was having a blast in Thor: Ragnarok (like every other actor), where she proved she could also be pretty funny. That's a useful skill if the Thor series is pivoting towards comedy, but she's also proven adept at drama. She's got a magnetic screen presence, whether it's standing out in the boxing drama Creed or HBO's sci-fi Western, Westworld.
Much of how exactly Valkyrie becoming Thor would play out depends on the events of Avengers 4. Thor: Ragnarok ends with Thor as ruler of Asgard and leading his people to a new home on planet Earth. Assumedly, this is where a proposed Thor 4 (or Ragnarok 2) would pick up. Perhaps Thor is too busy as king to be an Avenger and he becomes a mentor figure to Valkyrie. Maybe he dies in Avengers 4. Either way, there's a void to be filled and Valkyrie seems to be positioned for a big future in the MCU. It's rumored that she'll be appearing in Avengers: Infinity War, so hopefully this charming rogue can further prove just how worthy she is to be a key player in Phase 4.