At the end of the story, Natsu makes a difficult choice to go against Lucy’s wish for her sake. Even though Natsu knows that he would also end up carrying a heavy burden because of this, he makes that choice so as to protect Lucy. I think Natsu’s feelings for Lucy might go a little beyond camaraderie and friendship.
Natsu’s feelings for Lucy have yet to be clearly depicted in the original work, but I had Mashima-sensei’s consent to develop it a little in the movie. I tried to express Natsu’s feelings for Lucy, which go a little beyond friendship, through his gaze and movement.
So do I. When Lucy grows as a person, Natsu will also grow along with her. I’ll be happy if the audience pays attention to Natsu’s feelings towards Lucy. Even in the opening scenes where Natsu is fooling around, he is paying attention to Lucy and is the first to rush to her when something happens.