The Winter Soldier – the obvious answer

The most obvious answer is that the Winter Soldier is in that stasis tube as the main antagonists of the movie battle it out. After all, some of the most poignant images in the trailer show Bucky once again brainwashed, in full Winter Soldier apparel, even with some upgrades. And it’s totally understandable for people to indeed believe that he is in that stasis tube.

Except he isn't "“ frame by frame, that Civil War trailer scene shows Steve's pal in the background, trying to take Iron Man down, trying to help Captain America. Later on, his endeavor will prove fruitless, since he will have his metallic arm ripped off.

So, even with the Winter Soldier at the height of his powers, Iron Man is just too much for him. And if he isn't in that stasis tube, who is?