The contracts of Chris Evans and Sebastian Stan may hint at a major vulnerability. Evans is signed on until Avengers 3 and Stan is signed on for a total of 9 (far past Avengers 3)....if you know the Winter Soldier storyline, Cap dies and Bucky takes up the mantle. My guess is that Avengers 3 will be that moment...Thanos kills Cap (obviously a big moment but also shows to the audience just how powerful Thanos is) and Bucky steps in. The second cut-scene in Winter Soldier also leads me to believe Marvel is building towards that.