"Whosoever holds this hammer, if he be worthy, shall have the power of THOR."
These are the words inscribed on the head of Mjolnir at the command of Odin. While it is implied that Thor is worthy to wield the hammer, there is also the implication that others can wield its power as well, based on the presupposition that the hammer (or the Allfather, depending who you ask) deems them "worthy." That said, what quantifies "worthiness"? Well, according to comic book writer Walt Simonson, "This hammer is a killing weapon. It's used to kill Frost Giants and others. So, Superman couldn't pick it up, cause he's never going to kill anyone, and the hammer knows that. Captain America, he's too patriotic. He's too much a symbol of America to be chosen by this Norse artifact. So he couldn't get it."
I'm sure everyone's mind is flashing back to this scene (above) in Age of Ultron where the team takes turns attempting to lift Thor's hammer. We see every member of the team attempt to lift the hammer except for Black Widow. Captain America succeeds in budging the hammer, but not lifting it. Stark says "The handle's imprinted. 'Whosoever is carrying Thor's fingerprints' is, I think, the literal translation." To which Thor responds, "That's a very interesting theory. I have a simpler one. You're all not worthy." But I want to talk about someone else who IS worthy: Beta Ray Bill.
Now, there are some of you out there who have no idea who Beta Ray Bill is. Bill is a member of an alien race known as the Korbinites, and is actually the race's champion warrior. In the original comics, SHIELD picks up an alien fleet passing through our solar system and send Thor to investigate. Thor comes upon the sentient, advanced ship, Skuttlebutt, which perceives Thor to be a threat and awakens Beta Ray Bill from suspended animation. Bill and Thor battle to a standstill until reaching Earth's orbit, at which time, an enchantment over Mjolnir which returns Thor to his mortal form if he is separated from the hammer for more than 60 seconds, swings the balance in Bill's favor. No sooner has Thor been returned to his mortal form as Donald Blake, than he is knocked unconscious by Bill, who takes Blake's cane (Mjolnir's disguise when Thor is in mortal form), and strikes it on the ground, thereby giving him Thor's power and costume.
Bill travels to Earth, defeats a SHIELD scout team, and then is suddenly teleported to Asgard by Odin, who mistakenly believes he is summoning Thor. After some confusion, Thor is returned to Asgard, and Bill's origin is explained. Bill states that the Korbinites were nearly wiped out when their galaxy exploded, and as such, the survivors headed off in warships led by Bill and the Skuttlebutt. Discovering that Bill is worthy to lift Mjolnir, Odin proposes a death match between Bill and Thor in the realm of Skartheim in order to determine who should retain possession of the hammer. Both fight their opponent to unconsciousness, but when Bill awakens first, he is deemed the winner. Rather than killing Thor, Bill spares his life, telling Odin that Thor is too worthy an opponent to murder in cold blood. Bill is somewhat conflicted, not wanting to kill Thor, but at the same time realizing Mjolnir's potential for aiding his people. Finally, Odin proposes a compromise, giving Bill a different hammer, Stormbreaker, which has identical capabilities to Mjolnir and was cast at the same time. Learning that the destruction of Bill's galaxy was caused by a mutual enemy, Bill agrees to help defeat him, which he eventually does with aid from Thor.
Now that you are familiar with Beta Ray Bill's backstory, you must surely be wondering what makes a guy like that worthy to wield Mjolnir or its fraternal twin, Stormbreaker. Well, according to Walt Simonson, Bill was created, "because he's noble, and he's designed to kill. He's got a great purpose as a warrior, and also the noble ability. That makes him 'worthy' whatever that may be."
Now, I'm sure you're all wondering why I think an alien version of Thor should be in the MCU. I assure you, I do have reasons. First of all, the Korbinites are a species that I would absolutely LOVE to see added, and you can't add the Korbinites without including their fearless leader, Beta Ray Bill. Plus, I'm sure there would be some interesting banter between him and his Asgardian counterpart. Beta Ray Bill's powers and abilities include super strength, speed, stamina, and the ability to battle the mighty Thor (who is often cited as one of the most accomplished hand-to-hand fighters in the Marvel Universe) to a stalemate.