Re: How can they keep Cap 3 fresh?
Maybe introduce a heroic supporting character who Steve might clash with on ideological grounds? Like Josiah X, for example. Have him join up with Steve and Sam and butt heads with them over their respective world views, with Josiah being super cynical about what America stands for, Steve being super optimistic, and Sam resting somewhere in between. Might make the cast a little cluttered, I don't know. I just mostly like the character and would love to see him in a movie.
The world does not work the way you think it does: