Heading into [Captain America: Civil War](movie:994409), the death of one of our beloved heroes should only be used to move the story forward. We need to raise the stakes for our heroes, but we also need to head in the right directions with each character. The Writers, Directors, Producers, etc. for all the Marvel films, I'm sure have heavily considered the pros and cons of killing off characters. While I don't think any particular choice will be popular, the benefit has to outweigh the consequences.
Having said that, these are the most likely candidates for a noble, cinematic death, at least in my opinion:
8. Quicksilver & Scarlet Witch
Least likely of the candidates to perish due to their limited screen-time. It would probably be foolish of Marvel to introduce these characters as Ushers of "The age of miracles" and for them to suddenly just perish by the end of the film. From the looks of the trailer [Quicksilver and Scarlet Witch](movie:1081109) start off as possible bad guys. My guess is that there is something big in store for them beyond this film besides their ultimate redemption.
The Avenger with the least character development so far. Also the least number of film appearances (Thor Cameo not counted). With no certain future, Hawkeye could be on the chopping block. However, Marvel has stated that Hawkeye will be given more screen time and will be getting more back story in the coming films.
6. Nick Fury
Since his "death" in Captain America: The Winter Soldier, some would say it's probably too soon to kill him for real considering he is one of the driving forces behind The Avengers. There is probably more story to be told while he's "Off the Grid". Considering we've barely seen him since Winter Soldier, I'm thinking that they'd better have a damn good reason for bringing him back just to kill him.
5. Maria Hill
Maria Hill has played major supporting roles in The Avengers & Captain America: The Winter Soldier. She has also been in 2 episodes of [Agents of S.H.I.E.L.D.](series:722469). A force to be reckoned with within SHIELD, now working for Stark Industries. She has grown close with Tony & Pepper, and she is already a friend an ally of The Avengers. Now that SHIELD is no more, her fate is uncertain. Who knows? Her death could unite The Avengers in the way Phil Coulson's did.
Why Thor? Well, this would be more of a death in the style of Phil Coulson. He would be dead, but somewhere along the lines of, he's coming back to life soon, but the other characters don't know that. Considering we're in the "Age of Miracles", this could be a possibility.
3. Black Widow
First of all, as of right now, she is the only female Avenger. She is also an incredibly popular character. The Guys Want her, the Women want to be her. That being said, if you are looking at the most recent trailer for the film, it looks like Natasha is getting a greater back story in the form of flashbacks. This could be great for her character development. It could also be setting up her downfall. Countless films, shows, literature have used flashbacks as a plot device to further a character's story, or to show the beginnings alongside the end. Her death would royally piss off [The Avengers](movie:9040), especially Captain America causing him to split off from his fellow Avengers and lead into Civil War. This would piss off fans of the character, as well as comic fans seeing as she hasn't had a solo film fighting any of the villains she's faced in the comics.
2. War Machine
War Machine, even in the comics, is more of a sidekick to Iron Man. While popular, he shares villains with Iron Man and other characters in the comics, how much more use can he be? He's been in 3 Iron Man films, and now he's joining the Avengers. He's Grown as a hero but how much more can we get from him just being Tony's buddy, Number 2, and Confidant. Hos death is very likely because it would probably send Stark over the edge and lead him to face off against Captain America in Civil War.
Ultimately, in my eyes, the most likely Avenger to die is Vision. I know, we haven't even seen a glimpse of him in any trailer. They're obviously keeping him a secret. My theory is that he will be a one-off character and we will see him in this film, we will like him and ultimately heartbroken when he perishes. I really don't see much use for him (for cinematic purposes), after (presumably) defeating Ultron. I really feel that he will be a "one and done" character for this particular film Universe, although, if he's really in-demand, Marvel WILL find a way to bring him back.
There you have it, my theory as to who is most likely to die, if in fact, death is coming. The theme of Avengers 2 being "Death" could just be to mislead the fan base, or mean something else entirely. As long as the death makes sense to move the story forward, I think it'll be alright.