It was unfortunate to see the death of such a great character so soon. I was not expecting the death of Quicksilver in Avengers: Age of Ultron last year, but I have some theories on how he could potentially still be alive. First thing to note is that after he dies towards the end of the movie, the last time we see his body is when Hawkeye boards it in the S.H.I.E.L.D. aircraft. From there we are left to speculate.
S.H.I.E.L.D. And Project TAHITI
While he did indeed die, there are still some ways on how he could return. In the TV show Agents of S.H.I.E.L.D. we learned that Agent Phil Coulson, who was first introduced to the MCU in 2008's Iron Man, was brought back to life after his death in 2012’s The Avengers via project TAHITI. The project was created by Nick Fury and directed by Coulson to, in the case of a fallen Avenger, bring him or her back to life.
The project may have temporarily been dismantled in Agents of S.H.I.E.L.D., but since an incredible amount of information is compartmentalized within S.H.I.E.L.D. and with the new organization, the ATCU, led by General Talbot acting over S.H.I.E.L.D., the possibility exists.
Phil Coulson And Project TAHITI
We could think that Coulson, after experiencing an immense amount of excruciating pain when he was been reanimated under project TAHITI., would never want the project to continue, but knowing his deep respect for the Avengers, and him knowing of his special opportunity at a second chance at life, I am sure he would want the same for a fallen Avenger. Now that he is no longer the acting director of S.H.I.E.L.D. after the end of season three of the show, Project TAHITI can be something the new director could reintegrate.
United Nations And The Sokovia Accords
At the end of Captain America: Civil War, we are left with an Avengers team consisting of three members. Now that Tony Stark, Vision, and Rhodey Rhodes work under the supervision and control of the United Nations, with such a low hero count, could Tony and the U.N. have plans to bring back Quicksilver?
Because we don't know what happened to Pietro's body after the battle in Sokovia, the U.N. in collaboration with Secretary Ross, probably in the underwater facility we saw in Civil War, could be experimenting on the body, trying to reanimate it and potentially weaponize it, or something similar in the matter. If this then could be true, then why did Captain America didn't free him like the other heroes at the end of the movie, with the Russo brothers leaving us with a smile on our faces? A reveal like that is best suited for one of the Infinity War movies, and if that is the way Quicksilver is handled, then the Winter Soldier probably wasn't the only thing Cap brought to Wakanda.
Reanimation Is Part Of The Accords?
It could also be possible that one of the clauses within the Accords mentions what would happen to an enhanced individual if he or she was ever to die or suffer terminal injuries in combat. While this wouldn't apply to Quicksilver since he was already dead before the thought of the Sokovia Accords ever came to light, this could apply to his sister Wanda and Vision. Of course, the government doesn't have to mention their intentions in experimenting on enhanced individuals, as their "experiments for the greater good" are usually done in top secrecy.
The Magic Of Doctor Strange
Another possible way Quicksilver could return is via the mystic powers of Doctor Strange, but I’m not positive that Doctor Strange has that kind of heavenly power. Maybe if used in conjunction with the powers of Scarlet Witch?
We will first have to see how Marvel portrays Doctor Strange in his new movie this November. Depending on how similar Doctor Strange is on screen compared to how powerful he actually is in the comics, then a revival spell could be a possibility. I think it's more plausible for Quicksilver to return through project TAHITI or Secretary Ross, but a Doctor Strange connection is a thought we can’t discard.
More Than One Quicksilver: The Multiverse
Another way that Quicksilver could return, and this by far is my favorite way, is through the introduction of the Multiverse. If this should be the case, then it is also a way to connect the Fox X-Men Universe with the Marvel Cinematic Universe, in turn creating a Marvel Cinematic Multiverse.