In the comics it is written on Mjolnir that "whosoever is worthy to wield the hammer will have the power of Thor". If so who in the Marvel Cinematic Universe is really worthy of gaining the hammer and should Thor be defeated to lose the worthiness?
So far, on-screen, there are only two-ish people, that have been shown to be worthy to wield Mjolnir.
In the comics, there's a whole list of people (see, e.g. this scifi.SE question) who were worthy at one time or another, but so far we've seen no evidence that any of those people will be capable of wielding Mjolnir in the MCU.