I've been thinking a lot about "Schrodinger's Quicksilver." We know from multiple sources that Quicksilver was, at some point, intended to survive this movie, and that the decision to kill him may have come as late as post-production. I think there is proof of this in the original teaser.
Specifically, the famous shot of Wanda falling to her knees and screaming. Not only does this shot not appear in the movie, but the similar replacement shot is set in a completely different location (Ultron's doomsday church vs an open outdoor area). I think there were two different versions of the battle shot:
Version 1: Wanda fights the robots outside with everyone else. Pietro gets shot. Wanda screams and runs to his aid. She manages to save him.
Version 2: Wanda volunteers to guard the doomsday machine. Pietro gets shot. Wanda screams but can't help him without leaving her post. Pietro dies.
Version 2 is the one they went with.