Despite the new information and trailers we’ve gotten for Avengers: Age of Ultron, there’s still a lot of mystery surrounding newcomers Quicksilver and Scarlet Witch. Who are they? What’s their history? Why are they working with Ultron? However, the biggest question on fans minds is how they got their powers. The common theory is that the Marvel Cinematic Universe is making them Inhumans - and with the announcement of an Inhumans movie coming in the next few years, this doesn’t seem farfetched. Not only that, but if we look at all the details we’ve learned about these characters so far, it’s looking likelier that the Marvel Cinematic Universe could be setting Pietro and Wanda up to have a big role in the 2018 film
In the comics, the term "Inhuman" doesn’t just apply to the citizens of Attilan, where most Inhumans live. Anyone who is a descendant of the original humans that were experimented on by the Kree can become an Inhuman - and many of these people aren’t aware of their genetic history. Their physiology is already superior to a normal human’s, but it is exposure to the Terrigen Mist that awakens their inner power. Since the term "mutant" is off-limits to Marvel Studios, being Inhumans could explain how Pietro and Wanda have their powers in the MCU. For years they lived their lives like normal human, but somehow they were exposed to the Terrigen Mist and gained powers. Judging by the mid-credits scene for Captain America: The Winter Soldier, screencapped below, Hydra are probably the ones who ultimately expose the twins to the substance - or at the very least were able to figure out the source of their abilities.
Marvel Studios President Kevin Feige has previously stated that the twins won’t be Inhumans, but that was long before the Inhumans was announced, meaning that his comments could have just been meant as misdirection.
The Avengers: Age of Ultron San Diego Comic-Con concept art suggests that the twins will eventually help the titular team take down their villainous robotic enemy - but perhaps by the end of the movie, the twins may leave their new teammates to travel and learn more about their unusual physiology. This is where Inhumans comes in. Like with most actors who sign onto Marvel films, Elizabeth Olsen and Aaron Taylor-Johnson surely have multi-picture deals, so we can presume that Age of Ultron won’t be the only time we see them on the big screen. Feige did say we would see Easter eggs for the Inhumans before the actual movie comes out, so it’s possible that Marvel may be setting them up as our entry point into the 2018 film.
So how could this work? Perhaps the twins learn about Attilan and find their way to where the majority of Inhumans have been living for centuries. Rather than drop the audiences directly into this unusual society, Quicksilver and Scarlet Witch will be the perspective characters of the film. We’ll be following them every step of the way as they meet the royal family, discover where the Inhumans originated from and learn about this city full of people like them.
With all the talk of how important they are to The Avengers: Age of Ultron, it’s doubtful that Marvel would set aside Quicksilver and Scarlet Witch until the Avengers: Infinity War movies. Inhumans may be Marvel giving the twins a chance to have a bigger role in the franchise beyond just their identities as Avengers. While the Inhumans themselves can be the connecting point between Earth and the cosmic side of the Marvel Cinematic Universe, Wanda and Pietro can serve as the connecting point between the Inhumans and Avengers. Not only does this movie give an opportunity for Quicksilver and Scarlet Witch to shine on their own (relatively speaking), but they would be the ones who could bring in the main Inhumans to help the other Marvel heroes fight Thanos or any other giant threat that comes along.
Marvel must have big plans for these characters, and not just because they want to outdo the 20th Century Fox versions (they may not outright say it, but that competition is there). The Inhumans are a unique Marvel property, but if the film drops viewers straight into this weird society without any familiar context, it may turn some people off. Quicksilver and Scarlet Witch’s inclusion could easily prevent that.