(For those yet to see the film, I give fair warning that this article contains huge spoilers for the end of AOU!)
I'm going to start out by saying that this theory is based primarily on the character of Quicksilver and could potentially spoil the ending of the film for those yet to see it, so by this point you've had fair warning.
Quicksilver has always been a favourite character of mine, I've always found his brash and arrogant personality so relatable. His no nonsense attitude towards getting the job done and willingness to do whatever it takes to reach his goal whilst being overly protective over his sister are admirable qualities.
I was majorly impressed with how Quicksilver turned out for "Days of future past" and in my opinion that one scene stole the entire film, which I was surprised with considering i really wanted to like the MCU version better after all the pre released movie images. I feel Bryan Singer really blew fans away by exploring and depicting how amazing life would be from the perspective of a speedster, which sadly I feel was lacking from AOU. Admittedly I preferred the brooding a**hole version of the character as depicted by Aaron Taylor Johnson compared to the young goofy teen version played by Evan Peters, but both were portrayed exceptionally well by both actors, I just really wish AOU explored the characters power set a bit more visually like they did in DOFP.
By the final show down in Age of Ultron Quicksilver makes the transition from lackie of Ultron to blossoming Avenger, that is until he makes the dramatic sacrifice to save his budding mentor, Hawkeye from a barrage of gunfire from a desperate Ultron in a hijacked ship. In his dramatic last effort as a hero he manages to race to save Hawkeye, but in doing so was shot dead with his last words mime his first (in the movie that is) "you didn't see that coming?" Before collapsing to the ground. His sister Scarlette Witch senses this and erupts with a huge telekinetic blast after telepathically feeling her brothers death, obliterating an entire fleet of Ultron sentries in the process.
Now given that I really like both the actor and the character of Quicksilver from AOU, I was extremely disappointed when he died at the end. (It was like Joss Whedon had kicked me in the nuts two films in a row. The first being whatever the hell that was that happened to Hawkeye in Avengers Assemble.) This cinematic death and panic at the demise of yet another favourite character of mine led me to scour the Internet in search of how many films were in both Elizibeth Olsen's and Aaron Taylor Johnson's contracts with Marvel Studios, the end result of which was just multiple quotes saying "multiple pictures!" A pretty vague collection of responses from both actors in interviews.That's Half an hour of my life i will never get back. (so many Honest trailer videos I could've been watching instead.....)
But this vague collection of answers led me to speculate how Quicksilver could return given that their power set has been altered slightly for the MCU, according to their cinematic character bio's Scarlete Witch does not seem to posses her reality altering powers, (for now at least) this would seemingly rule out her raising him from the dead for the time being. And of course these theories will be under the assumption that Johnsons multiple picture deal wasn't just the end scene from Winter soldier and Age of Ultron.
Theory number one:
As previously mentioned above about the alterations made to the power set of the Scarlet Witch and her abilities being based mainly in the telepathic and hypnotic wheel house at the moment, things are going to be a little more difficult than just wishing her brother back to life. But given how close the twins are, I highly doubt she'll let this one go easily. (Although in the film we could've seen more of this bond, as we only saw a brief moment of grief from Wanda, we didn't even get to see what happened to his body afterwards.) But given the fact that Ultron expanded and grew from old alien technology from the battle of New York and the twins gained their powers from the sceptor of similar alien creation, I think there's a strong chance that their powers could grow and develope further. This growth and development in powers could lead Wanda to lean more towards the mystic arts and being an inexperienced young woman she's going to need some training in developing and controlling her expanding and growing new abilities. Enter Doctor Strange, master of the mystical arts, the sorcerer supreme. Doctor strange has a realease date for November 4th 2016, which would give the good Docotor plenty of time to get himself aquainted in the MCU long before the next avengers outting, or with the rumour mill cranking up the past few months we could even see these two mystical beings meet, lets say, around a certain civil war. Leaving a potentially large pathway for both character to develop and dive deeper into the magic portion of the MCU that is still yet to be explored, we could even see Wanda seek the tutolage of Doctor strange for the soul reason to tap into her latent magical abilities to raise her beloved brother from the grave.
Theory number two & three (elements kind of overlap):
My next theory that springs to mind stretches all the way back to the extremely hit and miss relationship shared between Quicksilver and a certain royal alien family of an "Inhuman" nature. With a November 2nd 2018 release date for the Inhumans film and their introduction being handled already in Agents of Shield and given the history of Quicksilver and the Inhuman royal family, I think this could be a strong way to go, given that the Inhuman film would land in the middle of parts 1 and 2 of infinity war. Agents of Shield are already handling the introduction of this alien race but we may not even meet the royal family until the film comes around, given Marvel Studios current skill at altering stories I can really see them working this one out.
Now I don't watch Agents of Shield (I tried, it just didn't feel right!) so I can only speculate from what I research online, but there's one of two avenues they can explore here regarding the characters shared history together. In one instance Pietro is wounded and is found by Crystal, a member of the Inhuman royal family, In the comic Pietro is nursed back to health by Crystal and the two later marry and have a daughter. Now given that Quicksilver is already dead in the MCU, I highly doubt any level of nursing could pull you back from that, but there is still plenty of room for the old Marvel Studios shuffle round to make the pieces fit. But if the Inhumans were to discover his body in some way, it would be a fantastic way to tie the Avengers in with the Inhumans, especially if Scarlet Witch thought her beloved brothers body had been stolen, it would be really fantastic to see the story line depicted where Crytal's uncle Maximus the Mad (brother to BlackBolt king of the Inhumans) uses their alien technology to turn Quicksilver insane and turns him against the Avengers. But again given how similar that plot line would be to Age of Ultron already (quicksilver being used against the Avengers.) I doubt they'd like to repeat themselves, although written well, could provide some excellent and compelling drama for the twins on the big screen.
which leads me into a similar theory, theory number 3. Comic book fans may remember the follow up story to House of M, Son of M featuring none other than Quicksilver. During this story Quicksilver wakes after the events of M day where Scarlete Witch had de powered most of the worlds mutant population, including her brother. In order to regain his powers Pietro exposes himself to terrigen mist and inserts terrigen crystals into his body without permission. Long story short, Quicksilver gets the crystals removed eventually after his powers expand allowing for time manipulation and increased speeds of up to mach 10. He is imprissoned and depowered once again after having the crystals removed, where he hallucinates and eventually inexplicably regains his abilities.
Now with some brief tweeking (given that they can't make any references to anything relating to Fox properties!) instead of Scarlete Witch changing the face of the world by casting her "no more mutants." Spell, she could (with the help of Doctor Strange) raise Pietro from the dead, he awakens powerless due to being an "enhanced" in this world and not a "mutant!" Seeing his sister flourish with her growing abilities, he becomes jealous and envious. The Avengers with the help of the Agents of Shield discover the Inhuman race and their royal family, where of course they uncover the terrigen mist and crystals (the source of the Inhumans mutations and abilities) after learning of the origin of this new species. A depowered Pietro tags along and naturally falls for his love interest Crystal of the royal family. During their interaction with the Inhumans the team learn of the uses of the terrigen mist and crystals and their location within Atilan (home of the Inhumans) Pietro then takes it upon himself to try to regain his lost abilities with altered and hightened results. (this process doesn't kill Pietro for the same reason him and his sister Wanda were two of the only surviving "volunteers" of Baron Struckers experiments with Loki's scepter, which as you find out in the contains an infinity gem.)
I really hope Marvel Studios brings this character back, both for the use a good actor and a great character. I feel really passionate about this character as a favourite of mine, there is so much exciting potential a speedsters has when being translated from page to screen, just look at the incredible job they did with their interpretation on X-men Days of future past and over on the CW's The Flash, there are endless possibilities and extremely incredible visual aspects to be displayed and explored from such an exciting, fast paced power set. Given the right writing and attention I feel the folks over at Marvel Studios could pull off something 10x as visually mind blowing compared to the result from Fox, and that was one pretty impressive piece of cinematography right there, but I have faith that the MCU could top it! (Biased much!)
I hope you have enjoyed reading my first attempt at jumping in with the speculation and fan theories and please feel free to let me know your own theories that have spun out of Age of Ultron, like where the hell was Thor's side plot looking for Loki's benefactor (which would've made the Thanos mid credit scene make a lot more damn sense. I mean Thor barely mentioned it and Joss mentioned that sub plot loads during pre release interviews! But I digress.) fingers crossed for the extended version of the film come release day.
I hope you've enjoyed the read and again, please let me know your own theories, as they are some of the most enjoyable and diverse reads from contributors that i've read on MoviePilot.