Re: Who dies?
I already gave my rationale for killing Hawkeye and Nick Fury, but how about killing off certain other characters who I see as potentially expendable.
Winter Soldier dying would be incredibly painful for Captain America to have to lose Bucky Barnes AGAIN and know that he's been killed off for real this time and is never coming back. It would make Cap 3 start with a tone similar to The Wolverine where Captain America has to deal with his grief and has gone from a courageous hero to a burnout who no longer cares whether he lives or dies. It's about his journey from despair to triumph where he realizes what it truly means to be a hero.
War Machine is a cool character whom people like but his death would shock the audience. Have him leave the Iron Patriot armor behind and then don the War Machine Armor one last time for one last fight as an affirmation of who he truly is. He's not a PR stunt, he's a soldier and after Captain America gives him a pep talk about going on that same journey, Rhodes sacrifices himself to stop Ultron.
Pepper dying because Ultron believes that humanity is flawed, inferior and deserving of death but having him still have an attraction to her and then building Jocasta in her image would truly get the audience to hate Ultron and see him as a monster. Then Jocasta not just being modeled after Pepper but having her brain inside would be really fun to see as Jocasta is forced to confront her past but accept that she's not actually Pepper as Pepper is dead along with Tony struggling with whether or not he's now in love with a machine.
Quicksilver dying would actually be expected. By that, I mean that based on what Joss Whedon said about Scarlet Witch possibly playing a role in Ultron's evolution, having Ultron then kill her brother would motivate her to join The Avengers. Within the Ultimate Universe, Wanda's death motivates Quicksilver to join Reed Richard's and Hulk as fallen heroes so having Wanda want to atone for her brother's death would be a cool way to develop her. I mean, have there ever been any great Quicksilver stories that weren't related to the X-Men anyway? With Fox doing their own Quicksilver, I can see Marvel getting rid of their version as he has limited potential for impacting any given story.