There's been plenty of rumours swirling that an Avenger is set to die at some point soon in the Marvel Cinematic Universe, and Thor's name has been tossed around in this debate more than a few times. Though it's highly unlikely that Thor will actually die in his next film, the upcoming [Thor: Ragnarok](tag:956858), not least because Hemsworth is still contracted for three films including Infinity War: Pt 1 & 2, it wouldn't exactly be a break in continuity if he did. Comic book characters have a bad habit of dying off without staying permanently dead, and Thor is no exception.
There's been a lot of speculation as to which storyline Thor 3: Ragnarok is going to follow, the most likely of which being the most recent occurrence of Ragnarok within the Marvel universe.
At the end of this particular cycle Thor enters into a limbo-like stage of hibernation, during which time he is believed dead by his fellow Avengers. Shortly after this Mjolnir is found on Earth and put under US Army protection, pretty much like what happened in Thor when the hammer was found in the desert. Doctor Doom later tries to take possession of the hammer but fails, and Mjolnir makes its way into the hands of a man carrying a bag marked "D.B" - Donald Blake, Thor's earthbound alter-ego. Blake eventually succeeds in combining the two personas again, bringing Thor back to life.
So, though it's unlikely that Thor will die, there's every possibility that in between Thor 3 and Infinity War, everyone's favourite Thunder God could go missing in action, especially if Asgard does end up being destroyed. This has happened several times in the comics, and it wouldn't be strange for it to happen here.
If so, what will happen to Mjolnir after Thor 3?
Thus far in the MCU the only other hero we've seen lift the hammer was Vision, in [The Avengers: Age Of Ultron](tag:293035).
This scene is amusing as it recalls earlier in the film where, at a party, all the Avengers try to lift the hammer and fail (with the exception of Steve Rogers (Captain America) who manages to shift it a little, much to Thor's displeasure).
However there's been a number of other characters in the Marvel Universe who have wielded Mjolnir at some point or another. Most of them have not yet made an appearance in the MCU, so here's a quick run down of who would be most likely to lift Mjolnir in Thor's absence.
Beta Ray Bill
Beta Ray Bill is one of those characters that seems unlikely to be adapted in the MCU, but then everyone thought that about the Guardians of the Galaxy and look how that turned out.
Beta Ray Bill is a horse-faced alien who has been around in the comics since the 80's. Though he appears monstrous Beta Ray Bill proves himself to be a great hero, and so is the first non-Norse Marvel character to be deemed worthy of wielding Mjolnir. Of course Thor isn't too happy about this to begin with, so after a while Beta Ray Bill is given a hammer of his own, Stormbreaker, and they both get along much better after that, becoming closely allied.
Beta Ray Rill also appears in the most recent Ragnarok storyline (which I mentioned above) in which he is saved by Thor, so you never know...
Thunderstrike / Eric Masterson
Another character who has served as Thor, Thunderstrike A.K.A Eric Masterson, has also been around since the 80's. Initially human, Eric Masterson was an architect rescued by Thor and given his powers in order to save his life after being mortally wounded by the supervillian Mongoose.
Masterson and Thor get separated and merged a few times; in one such instance Thor is banished. He gives Masterson possession of Mjolnir, and he joins the Avengers to serve in Thor's place. As part of the Avengers, Thunderstrike is present during the Infinity Gauntlet storyarc, which ties the character into the MCU.
Thor eventually defeats Eric and reclaims Mjolnir; similarly to Beta Ray Bill, Thunderstrike ends up getting his own mace from Odin in order to reduce competition for Mjolnir.
Back when it was announced that the new Marvel Comics Thor was a female there was some confusion in the non-comic book reading community. Had Thor finally conceded to wearing his mother's drapes? Did Donald Blake get a sex change? However, as is common knowledge by now, the 'female Thor' is not literally a female version of the Asgardian, but rather Jane Foster wielding the legendary hammer and in doing so gaining the powers of Thor.
Jane Foster lifts Mjolnir after the Original Sin story arc in which Nick Fury tells Thor a secret that renders him unworthy to lift the hammer (like in Thor when he finds Mjolnir in the desert, and is unable to lift it due to the actions that caused his banishment from Asgard). After witnessing her wielding his power, Thor concedes his title to her, though he is unaware that it is Jane Foster who has taken his identity due to the fact that her mortal form is weakened; currently she's undergoing chemotherapy for breast cancer.