One of the most astonishing scenes of The Avengers: Age of Ultron was the death of Pietro Maximoff a.k.a Quciksilver. Chances are he may return in future Marvel Cinematic Universe movies, as Aaron Taylor Johnson has hinted. The likelihood of his return to rejoin the Avengers can be deducted on the following grounds:
GH.325 is a drug that was synthesized from an alien corpse by S.H.I.E.L.D. It revives dead tissues in one’s body. However, it comes with strange side effects like hypergraphia (subconscious drawing of diagrams in one case), aphasia (language disorders caused by damage to the brain), catatonia (a kind of behavioral abnormality) and psychosis (loss of contact with reality). After Phil Coulson’s death in The Avengers, Nick Fury used this drug during a series of painful medical procedures to bring him back to life. It regenerated his heart tissues apart from healing other injuries.
Treating Quicksilver with this drug could be an excellent way to connect Agents of S.H.I.E.L.D. with the MCU. However, it is somewhat unlikely as no one would want Pietro acting wacko all the time due to the drug's side effects.
2. Doctor Strange
Dr. Stephen Vincent Strange is one of the most powerful sorcerers. He will soon be coming to the MCU in the movie Doctor Strange (2016). A few of Vincent’s magical powers include energy projection and manipulation, matter transformation, animation of inanimate objects, teleportation, illusion casting, mesmerism, thought projection, astral projection, dimensional travel, mental possession and time travel. All of this makes me believe that this guy would be awesome. After his appearance in the MCU, we might see a lot of time-related mumbo jumbo. His travelling back in time might be good news for Wanda Maximoff. Doing this would make the events of Age of Ultron pointless however.
3. The Regeneration Cradle
The Regeneration Cradle was used to create Vision in Avengers: Age of Ultron. This technology creates tissues as Dr. Banner claims. The team might be asking Dr. Helen Cho for help regarding Quicksilver’s death. This is, in my opinion, the most probable way of undoing Pietro’s death. I think the cradle has been introduced mostly for this purpose.
4. Time Gem
The Time Gem is one of the six Infinity Gems. It enables its possessor the ability to travel through time, and manipulate time through acceleration or slowing it down. The Gem can also be used to age a person or revert them back to their infant state. It still has to be introduced in the MCU. It will most likely be in the hands of the mad titan Thanos in The Avengers: Infinity War. But once in safe hands, it may be used to bring Pietro back to life.