Quicksliver is one of the most important characters in comic book history. But this was not the case for the MCU as he died in Avengers: Age of Ultron while trying to save Hawkeye and a Sokovian boy from being killed by Ultron. Losing Pietro was a big moment for his sister Wanda Maximoff that has a significant impact on her character act. If he had survived the round of bullets and she did not suffer that loss, their relationship would have grown more during the course of their 3 appearances.
Without that need, Both she and Pietro would have joined the team, but they would still have to rely on each other more than the rest of the Avengers. Quicksliver's relationship with the other Avengers did not have much screen time, but it deserved to have more focus. Captain America and Pietro would both have a falling out over their differences of being the protective big brother of Wanda, which would lead him to joining and eventually betraying Iron Man's side in Captain America: Civil War. It would also show that despite battling each other and unlike Tony and Steve, nothing would ever break their love and loyatlity to each other. Even though they would be imprisoned at the end of Civil War, they would know that the Avengers reformed and they would rejoin the team when they need each other the most. I'd imagine if he survived beyond AOU, he would just get badly hurt and Wanda would still have had the outburst. He would just be a sitting duck, laying injured on the ground.
His costume would be an amalgam of his classic and Ultimate costumes, but with additional stripes on the front and back and boots. He would also have fingerless gloves and pads on the shoulders and legs.
Civil War: He would be on the Lagos mission with his sister, Captain America, Black Widow and Falcon, helping Wanda getting the smoke out of the building, stopping Crossbones's men by walking through walls and now outrunning bullets(compared to Avengers: Age of Ultron) and getting civillains to safety with his speed. He would have a harder time to put his hatred for the Stark family aside and join Iron Man’s side because he would be convinced by Tony Stark that it would stop enhanced people like his sister from causing causalities again. He would try to beat Cap again and run around Giant-Man in circles, but he would be defeated easily.He would also refuse to fight back against Wanda and as a result, she wins the battle, twisting his famous catchphrase back at him "We both didn't see this coming." He would not be happy about Stark placing his sister under house arrest, but Wanda would convince him not to break her out because it would violate the Accords. During the beginning of the battle between the two teams of Avengers, He would also run faster than everyone else, even Spider-Man during the beginning of the battle between the two teams of Avengers. He would appear as a blur compared to everyone else. I imagine the Lagos incident would still happen even if he drops Wanda to lift Crossbones up into the sky. He also would boast about Vision soon becoming his brother-in-la(when he battles the latter),have Black Widow help him learn of American culture, have a fallout with Cap about their differences of being the big brother to his sister and give Spider-Man a run for his money. At the battle in Germany at the airport, he would bicker and fight with his rival Hawkeye once again, while being surprised how he named his newborn son after Pietro and finally, when Wanda is attacked by War Machine, he finally has enough of the Accords restricting the Avengers even if it meant hurting his sister and turns against Team Iron Man, only to be knocked out by Tony Stark(who at one point in the film would name him "Super-Fast Bullet)", using a sleeping dart made from one of his missles. This would show that unlike Tony and Steve, nothing would ever break their love toward each other and even though they were on opposing sides, they would always protect each other if enemies were to hurt the other. He is later imprisoned at the Raft. In the end, he is freed alongside the rest of Captain America's team. Quicksilver's total screentime would be a total of 9 minutes which would change Captain America: Civil War's runtime to 155 minutes.
Infinity War: Pietro would be part of Cap's rogue team, sporting longer hair similar to Superman by the time of "The Death of Superman" storyline, He would also have a scene with Wanda about marrying Vision as he jokes she could've dated a microwave oven and being in love with one another(as a reference to the Ultimate Comics where they were in a husband-wife-like relationship) and react how Rocket Raccoon can speak. And the latter would say “Guy's like a Grosgumbeek Engine.” He would help his sister, Vision, Black Widow and Cap fight Corvus Glaive and Proxima Midnight except everything would basically remain the same. By this time, he would be capable of reaching supersonic speeds between Mach 8 and Mach 10. He would also outrun everyone else again, passing like a blur and create 3 duplicates of himself to help kill some of the Outriders during the Battle of Wakanda, surprised how big Wakanda is, even calling it similar to Narnia and been kept busy similar to the final battle of Avengers: Age of Ultron.Not to mention, he would also be busy dealing with the creatures (similar with during the final battle of Avengers Age of Ultron). Later, he would help Cap escort Wanda and Vision to a safe place, admitting even with his speed becoming more powerful as he becomes older, "The big, bad Vader strongman " would not stay down and try to attack Thanos, but he would freeze the speedster and beat him down, being surprised how fast a human can be and senses he has the potential for limitless speed. Not to mention, like Wanda, he would also lose his accent and witnesses Wanda being erased during the Snap when he staggers towards her - moving slowly for what feels like the first time in his life,calling her name a few times softly, now understanding how much Vision meant to her and when she begins to disintegrate. She looks back at him, acceptence flashing across her features. And he uses what's left of his energy to try and grabd her, but she disappears before he could get her. And then he weeps sorrrowfully and grieving for her. Avengers: Infinity War would've ran for 165 minutes as a result of him being in the film for 16 minutes.
Endgame: Pietro would assist the Avengers in their assult on Thanos. When he says he destroyed the Infinity Stones, he would say "You are not so right in the head.", thinking that he is lying. 5 years later, now sporting a new silver buzzcut and full-on beard, he would report that he is helping the remaining Sokovians move on from those who have dusted and formed a grieving council in honor of his dead sister. When the Avengers reassemble, he would arrive in less than 5 seconds, saying his most famous catchphrase "You didn't see that coming?". Later, when the team travels through time to collect the Stones, he would assist Iron Man and Ant-Man in retrieving the Tesseract in the aftermath of the Battle of New York, only to be knocked down alongside Tony when the 2012 version of Hulk accidentally attacks them, smashing the doors that led to the stairs he was forced to walk down and Loki uses the Cube to escape with Pietro commenting "This is so bad". After the Avengers return to their proper time, Pietro would comfort Hawkeye on the loss of Black Widow. He would also talk to him about bringing his sister back as he says he now feels like that she is capable of taking care of herself and he would do anything to bring his family back as well, showing that he now settled his differences with his long-time rival and they have now become friends. When Thanos travels to the present, he gets stuck under the rubble alongside Rocket, War Machine and Hulk and tries to vibrate himself and his teammates out, only to be slowed down by the rising water. Later during the Battle of Earth, he would be busy dealing with the Outriders and fight Corvus Glaive who is defeated and tripped by the speedster due to him now reaching between Mach 13 and 15. He would also help Iron Man, Captain America, Thor and Captain Marvel do more damage to Thanos, but he would not have been able to stop the Mad Titan sooner as Thanos would get one good hit on the speedster and knock him out. In the end, he tells Wanda how he wishes he could tell Tony about how much of a hero he was and that he could've become friends with him, showing that he lost all hatred for the Stark family. He would also go to Tony Stark's funeral. When Scarlet Witch begins to leave the Stark Cabin, they would have a callback to Age of Ultron in their last conversation about Pietro promising Wanda that he will come back for her whenever she needs him, while supporting her because she's still mourning Vision's loss and bickering "You know, I'm 5 years older than you". Wanda would respond quietly "Go". If Quicksilver had 20 minutes in the film, Endgame would go from 181 minutes to a ground-breaking 201 minutes.
Please let me know what your thoughts about Quicksliver having a bigger role in the MCU.