I have not read the comic series, but I heard that Crossbones is the character that kills Steve Rodgers. If that is true, this could be Chris Evans' exit from his Marvel obligations, and it would be one hell of a moment on screen. It would also follow into other characters that donned the red, white, and blue suit, like Bucky Barnes, and Sam (The Falcon).
I have absolutely loved the way Evans plays Cap, but I knew it would not last forever, and passing the torch as it was done in comic canon is not a bad way to go.