Disclaimer: I do not "know" who is going to die... This is just my critical theory. Alright, moving on. Possible spoilers ahead.
Joss Whedon is known for being a murderer of main characters. All fans of his are warned against getting attached to characters because its a well known fact that he enjoys shocking audiences with the deaths of characters that are otherwise guaranteed life. This knowledge has led to many people wondering who will die in Avengers: Age of Ultron. Rumors have basically flown around every member of the Avengers (except RDJ because, c'mon, he's awesome) but no definitive fan theory is gaining momentum. Now, mine may have many holes in it, but when looked at as a whole I feel it is the most realistic.
Originally, I was under the assumption that Captain America would die. It seemed the least likely to me and that was the driving factor behind my idea. It would come as such a shock that audiences wouldn't have time to react. My first theory, obviously, is wrong. We know he can't die because of the recent announcement of Captain America: Civil War; not to mention the fact that we know Chris Evans has many more films left in his contract.
My second theory was Hawkeye or Black Widow. They, despite what some adamant fans protest, are the most expendable members of the super team. They are not only not-superpowered, but are also not that unique when looking at the S.H.I.E.L.D members all around. Sure they have their specialties but the death of one of them (especially you, Hawkeye) wouldn't cause much of dent to the team's effectiveness. That's why I see this option as one of the least likely. Whedon wants to hit you where and hurts and although the death of appealing Black Widow would limit the girl power of the team, it wouldn't be that big of a surprise. Even Hawkeye will survive because he hasn't been 'himself' for the majority of the character's screen time.
No, Whedon wants to hit you hard. He wants you to be so blown away that this character could die that you would take to MoviePilot and complain about it (not to mention cry about it on Tumblr).He can't kill Cap. He can't kill Stark. There's a strong likelihood that Bruce Banner will leave the team during the film, as seen in the teaser trailer, so he can't kill him. Everyone else is too new to the team-- Vision, Quicksilver, Scarlet Witch-- or too expendable to deserve a death. That leaves only one person... The Mighty God of Thunder, Thor son of Odin!
Yes, Thor is going to die in Age of Ultron. The beloved Norse Titan and heir to the throne of Asgard will meet his end at he hands of the adamantium, vibranium, death machine named Ultron. Thor, the muscle-packing and founding member, will die. Now you might be questioning me pretty hard right now. How could Thor die if he is nigh invincible? How could Thor die if there's a third film confirmed for [Thor: Ragnarok](movie:956858), in 2017? How could Thor die if he has a contract to finish three (four?) Avengers films?
First of all, I know Thor will die because for one thing, he's the only left with enough of the shock factor to die. As I've pointed out the rest of the Avengers either have 100 certainty of survival or are too expendable to create a good enough impact. He will die at the hand of Ultron as teased in the trailer by his life-less hand beside the shattered Captain America shield. Whedon likes to do this. He distracts you with the broken shield, drawing all of the audiences' attention, but actually revealing the biggest secret of the film. But how can a God die? Ah ha, you see Whedon has been pulling our strings (not unlike the teaser trailer) this whole time. Remember in the original Avengers when Loki says that people think Asgardians are immortal, even Godlike, and can't be killed. Then he's like, "let's test that theory", and sends Thor to his plummeting doom. Of course Thor survives this easily but it does beg the question, with enough force, can he be killed? Could, perhaps, an indestructible force of its own-- Ultron-- have the power to kill a God? Whedon seems to hint so...
Secondly. Thor, according to the comic book timeline, is missing-in-action or dead during The Marvel Civil War. Did you hear that? That means that by the time Cap 3 comes along, Thor has to be out of the picture. He either has to die in Age of Ultron, [Ant-Man](movie:9048), or during Civil War itself (or busy with Loki in Asgard but that seems the most unlikely). Of those choices, Ultron easily seems like the most likely spot where Thor will meet his end. That way he will have the same involvement in the Civil War as he did in the comics, as a cyborg Thor... named Ragnarok.
Now, all the corners are coming together. Thor will die in Ultron, become the robot/cyborg Ragnarok-- created by Stark-- in Civil War, lend the edge to Stark that he needs to win the war, and then appear in the film named after the cyborg in 2017. I have no clue what Thor 3 could be about but I'm just guessing it has something to do with the real Thor returning from the dead (as nearly all comic characters inexplicably do, perhaps at the hands of Dr. Strange) to fight Ragnarok. Loki, who is confirmed for Thor 3, could possibly use the robot Thor to convince the people of Asgard to trust him again. When the actual Thor returns, Loki could use the cyborg to take out Lady Sif and the Warriors Three (as seen in the comics) and then have an cinematically epic battle between Chris Hemsworth against Chris Hemsworth! Jeez I hope I'm right because that would be epic.
Finally, to finish off my MCU theory, Thor will return alive in Avengers: Infinity War to reunite the now splintered superhero team. As rumored, Cap will create his own Avengers team as well as Stark creating his own team, but the arrival of once dead team mate will unite them as a universe just in time to battle The Mad Titan himself, Thanos.
However, the moral of the story is that Thor will be killed during 2015's, Avengers: Age of Ultron, either at the hands of Ultron, his warriors, or maybe even Hulk... but that's a different theory. What do you think? Who will really die in Age of Ultron? What is the future for the MCU?