In Marvel comics, death in comics is a pretty common occurrence. After all, Death is a comic book character, like an actual personification of death. Thanos is in love with her and Deadpool has dated her. It's a whole thing. The Marvel Cinematic Universe will likely start reflecting that side of comics more in Phase 3, and start having more fatalities in the line of duty.
Heroes are likely to start dropping in Marvel movies for a multitude of reasons: Contracts are expiring, storylines are coming to an end, more actors means more scheduling conflicts, it's logical for legacy character purposes, and death will raise the dramatic stakes and emotional weight of these films. Whether or not they'll stay dead is another story - this is the world of comics. Even the characters listed here with a low chance of revival are still probably coming back because that's how comic book mythology works.
So here's a Marvel Studios Phase 3 death pool (not Deadpool; that's something else entirely), making educated guesses as to who might be pushing up daisies by the time Phase 4 rolls around. Vote up the characters you think are most likely to die by the end of Avengers: Infinity War - Part II.
Why He Might Be Doomed: If anyone could survive seemingly everything, it would be this all powerful god-like cyborg. Unfortunately, he has one of the Infinity Gems in his forehead (seemingly as his main power source) and we see Thanos rip it out in the trailer, which doesn't bode well ofr the Vision.
When Will He Bite the Dust: Avengers: Infinity War - Part I.
Odds He Will Return From the Dead: Medium to high. Pop that super gem back in his forehead and just power Vision back up.
Why He Might Be Doomed: He's lovable, he's been around since the beginning (sure he was Terrence Howard for a second, but still), and his demise would cause turmoil in the ranks. And killing him off after paralyzing him would be a knife in the wound (though, his inevitable post-paralysis comeback makes his death less likely).
When Will He Bite the Dust: Avengers: Infinity War Part II.
Odds He Will Return From the Dead: Low. He's a fantastic actor and a fantastic character, but the only logical reason to bring him back (if they kill him) is to take up the Iron Man mantle, and as long as Robert Downey Jr.'s still showing up, that doesn't seem too likely.
Why He Might Be Doomed: He already all but predicted his own death it in Avengers: Age of Ultron. Plus, Idris Elba is a busy actor who has never quite had enough to do in the Marvel movies.
When Will He Bite the Dust: Avengers: Infinity War - Part I
Odds He Will Return From the Dead: Low, unfortunately.
Why He Might Be Doomed: He dies in the comics as a direct result of the Civil War storyline, and Evans has one more movie left on his contract after the first Infinity War movie. Plus, there are two potential temp Caps in Falcon and Winter Soldier, both of whom have thrown the shield as Steve's star-spangled replacement in the comics.
When Will He Bite the Dust: Avengers: Infinity War - Part II.
Odds He Will Return From the Dead: Low to medium. If he does indeed kick the bucket in Infinity War - Part II, there's always a chance they could bring him back, but Evans has talked for years about wanting to move on to more personal projects.