Recently, I spoke to Forbes contributor Mark Hughes about what it would've been like if Quicksilver made it out alive in Avengers: Age of Ultron and he explained to me if that happened, some things would have been quite different in Captain America: Civil War, Avengers: Infinity War and Avengers: Endgame. Although not as much as people would expect.
I asked him this on Quora and I realised that he thinks he wouldn't side with Tony Stark (Robert Downey Jr.), instead being on Captain America's team with his sister Wanda Maximoff. He also believes that he would've survived the Snap so that he could mourn his sister's death. Quicksilver may not have been entirely effective at stopping all of his opponents in Age of Ultorn, but he would only become stronger and faster up to the level of Evan Peters's Quicksilver in the Fox X-Men films. This would've showcased his potential for a Quicksilver series starring Aaron Taylor-Johnson as the character on Disney Plus.
I will write down Hughes's description how the MCU version would've been part of Civil War and Infinity War.
"I think in Civil War, Quicksilver would've joined his sister Scarlet Witch on Captain America's team, and would've been able to catch the fleeing mercenaries with Black Widow. But that would mean he wasn't nearby when Crossbones detonated his suicide bomb, so Scarlet Witch still would've had to stop the explosion as she did, and the result would've been mostly yhe same. Quicksilver might've been able to save a few people from the building as the explosion happened, but the key thing to remember is that his super-speed doesn't make his MIND think at super-speed, so his reaction time still is limited to how quickly he perceives a threat, thinks of a response and THEN uses his speed to carry out that response. Thus I believe he'd have only saved a couple of people from the explosion at most, while most of the victims still died.
At the UN, when the bomb detonates and kills King T'Chaka, it's doubtful Quicksilver would've been present to do anything to stop it. So with the Sokovia Accords passed and Bucky labeled a terrioist, it seems inevitable that Captain America would still go rogue to find Bucky by himself, meaning Quicksilver wouldn't factor into this part of it. Quicksilver would surely be at his sister's side, while Scarlet Witch was sequestering herself at Avengers headquarters. But later, when she wants to leave and Vision stops him because she's on a sort of "house arrest, Quicksilver would've reacted badly to the attempt to hold his sister.
Most likely, he'd either run away with her and then bring her back after she convinced him it was a mistake to leave, or she'd convince him immediately before he even ran off with her. Either way, events still would basically remain the same as in the finished movie at this point.
When things change is during the battle at the airport, during which Quicksilver could've easily outmaneuvered many of Iron Man's team and stopped their attacks. The most important moment would still be his ability to help Captain America and Bucky get to the jet to escape from the hanger as happened in the film anyway. But the difference would be this -- since Quicksilver could easily get Cap and Bucky to the jet without any trouble while the rest of the team fought, Scarlet Witch wouldn''t have been involved in that part of the situation and thus wouldn't have been injured which in turn wouldn't have distracted Vision and he wouldn't have accidentally shot War Machine out of the sky and broken his back legs.
After Cap and Bucky escaped, Quicksilver probably could've gotten to safety from the fight. It's unlikely he'd have helped other heroes from "Team Cap" relatively close to the fight and thus it wouldn't be hard for Iron Man's team to locate and catch several of them. So my guess is, Quicksilver and Scarlet Witch escape the airport battle and plan to meet up with Cap later".
Unless, Iron Man used his laser beams to knock Quicksilver out of the battle and War Machine's cripplement and Vision's distractation would've still happened. And he and Wanda would both be imprisoned in the Raft and unable to use their powers due to the collars that prevents enchanced people from using them, only to be freed by Cap at the end of the film.
"This would set up Quicksilver being on Cap's rogue team in Infinity War. The first battle between Cap's team and Thanos' Black Order wouldn't really change much, since Cap's team won that fight quickly anyway. Later, the team go to Wakanda and have the big fight with Thano's larger army, so Quicksilver's role in that would surely be to help clear the field of opponents. There were so many of those Outrider monsters, it would've kept Quicksilver busy for most of the battle (similar to the fight against Ultron's army of robots in Age of Ultron.). So again, I don't think much would've changed in the story despite Quicksilver's presence.
Where Quicksilver possbly could've changed the story in Infinity War is at the end, when Thanos himself arrives to kill Vision and steal the Mind Stone. But at that point, Thanos had enough power to simply freeze Quicksilver in place or kill him easily anyway, as soon as Quicksilver made his move. Remember, against less powerful foes, Quicksilver often was beatable and could be hit, tripped, etc, so against Thanos with the almost-full Gauntlet, I don't see how Quicksilver could've done much to change the outcome.
Would Quicksilver survive Thanos' "snap"? I think if Marvel had allowed him to live in Age of Ultron, they'd have kept him alive in Infinity War so he could mourn his sister's death."
This would've been similar to Scarlet Witch having the outburst that killed Quicksilver. Perhaps, if he just got badly hurt, the outburst and Quicksilver's loss for his sister would've been a parallel between Age of Ultron and Infinity War. Perhaps, one day, Marvel may revive him in the near future in a possible MCU adapation of the X-Men storyline Days of Future Past.
What do you think of these updated theories?
https://www.change.org/p/the-walt-disney-company-marvel-studios-bring-quicksilver-back-to-life-in-the-mcu