I don't think the guy with the shield is John Walker/USAgent. Why? you ask, simple Walker would be the first guy for registration of heroes and to go after heroes who refuse to register! Thats how blind he is, all for glory, none for honor.
My guess is that it's Steve Rogers/Captain America, of course he can't go by C.A. anymore so my thought is he returns to his "NOMAD" identity without the old costume.
That's my thought.
MM