So according to speculations based on the trailer and the comic, in AoU, the Avengers are going to see their worst fear or their hopes (which would explain the scene where Tony sees the bodies of the Avengers: that’s his worst fear)
Let me repeat that.
The Avengers are going to see their worst fear or hope.
Now. Just imagine what Steve’s vision will be.
In both cases it has to be Bucky. It can’t not be Bucky.