(Spoilers follow for The Avengers: Age of Ultron)
For any of you have seen the movie or even read a summary you're familiar with the scene where Elizabeth Olsen's Scarlet Witch performs a quick spell on Tony Stark and gives him a vision of his worst fear; the entirety of the Avengers, strewn about a platform, dead or dying, because of his inability to save them.
It's worth noting that there's a Black Panther easter egg here too with this image
Which shows the back of Black Panther's mask facing camera and what might be his clawed glove lying next to it.
The more interesting shot here to me is this however
Which shows a clearly injured Clint Barton, still sitting up as if ready to get up and keep fighting (a brilliant allusion by the way to just how determined and strong a character Hawkeye is) and laying next to him is the body of another man.
Who Could He Be?
1) Winter Solider
There's not a whole load of characters this could be, I've heard mentions of Winter Soldier due to the shiny arm but Sebastian Stan wasn't credited for a cameo in IMDB unlike the other characters like Heimdall and Peggy Carter who were featured in Visions and in the castlist. It's also worth noting that we see this character's right arm, Winter Soldier's mechanised arm is his left.
2) Shield Agent
My money's on a random Shield agent, given the mildly armored demeanour and unrecognizable actor I think it's likely there were one or two shield agents on the "mission" with them when they get killed. However given the extent of the universe now maybe we're seeing an incarnation of a famous shield agent from the comics such as Clay Quartermain.
Yet to be Introduced Character
There are a number of films in the lineup before we see when this scene would "play out" so to speak or when this future would occur. Among them are Ant-Man, Doctor Strange, Cap 3, Thor 3, Black Panther, Captain Marvel and the Inhumans.
Seeing as how the character doesn't resemble any already cast actors in the upcoming films we have to look a little further to the end of Phase 3, my guess might be on Captain Marvel, given how little we know about the characters featured in the film beyond Carol Danvers herself, the character is really important for the MCU as it's to be the first female-led film and we've looked at a lot of the earth between the Iron Man and The Avengers films, Guardians of the Galaxy taught us a lot about the outer Universe but Captain Marvel will give the studio a chance to connect the Earth to the rest of Space and an important part of Carol Danver's origins as Captain Marvel/Ms. Marvel is her relationship with Noh Varr, a Kree warrior who holds the mantle of Captain Marvel prior to Ms Danvers. Although Kree have a blue skinned physiology, Captain Marvel is always depicted as being a blonde, caucasian so maybe they're choosing to represent Varr as a human looking soldier for espionage purposes?
Anyway this is all speculative and my personal opinion but that being said feel free to leave any other ideas in the comments, Thanks for reading.