This ones a bit out there, but what if Mjolnir is powered by an Infinity Stone? It would explain the power of Thors hammer, how its able to tell if hes worthy or not and has the ability to revert him back to a powerless state. The hammer also bears similarities to the Orb, Sceptre and Tesseract in that it could conceivably house a Stone inside of its powerful outer shell. The hammers origin certainly support this notion; it was forged in the heart a dying star, one consumed by a black hole, and thus bears a relationship to a singularity... exactly as the Infinity Stones do. There's cause to disbelieve this one, however, because theres no doubt a reveal like this would piss off a number of fans. It goes directly against what the comics have always said and makes Thors worthiness and power less impressive in retrospect. But it's important to remember that this is a company that took The Mandarin we loved and brutalised his legacy, so anything is possible, even something as crazy as Mjolnir housing an Infinity Stone.