Now if anyone has read the Captain America comics, you would noticed that Steven Rogers the original Captain America has been killed in one of the stand alones and after his death, his friend Bucky Barnes the Winter Soldier took over the mantle as the new Captain America.
So if Winter Soldier survives in the Captain America sequel, he would return in Avengers Age of Ultron, he at the end of the film would become the new Captain America just in case Ultron kills Steven Roger. After all without Captain America, how would the Avengers know what to do in battle? Plus Sebastian Stan was mentioned to appear in 6 more Marvel films after Captain America the First Avenger so if Bucky does survive in Captain America the Winter Soldier, he will indeed return for Age of Ultron taking Steven Rogers' place as the next Captain America, clearing his name of the wrongs he was manipulated on doing, and helping the Avengers take on Thanos and other kinds of threats.
But if Winter Soldier does not survive in the next movie, then Joss Whedon maybe intended on having Ultron assassinate the Black Widow. After all Whedon was talking big on Scarlett Johannson's role in the Avengers sequel that her role would be significant meaning that the movie has her as the main character, that she would do something big to save the day, or be the Avenger for Ultron to kill off.
So as you can see that Captain America (Steven Rogers) and Black Widow are so far the most speculated choices that Joss Whedon is intending to kill because with Robert Downey Jr. signed on for the third Avengers movie, Chris Hemsworth for the next Thor movie, Hulk getting a sequel sometime during MCU Phase Three which is also to feature more of Hawkeye, and with Scarlet Witch and Quicksilver joining the team as its youngest members, who else would Whedon choose to sacrifice from the MCU?
So with Steve Rogers and Black Widow as the two specified targets and victims for Ultron, the question to which of them it will be is in the hands of the Winter Soldier's fate in Captain America the Winter Soldier
To find out, stay tuned for the movie when it hits theaters April 4th 2014.