I had to pare down the original question a bit, but I think that the discussion of Expressionism will still be relevant. On the outset, I think that both works were written outside of the Expressionist movement, which seemed to be more of a construct of Europe. That being said, I think that we can see Expressionist ideas still present as there is much in way of thematic convergence between Modernism and Expressionism, and both dramas are written with a Modernist slant present. For Williams' work, the Expressionist theme of regeneration, or new growth, lies in the Laura character. Laura is living in the shadow of her mother and the intense antagonism she shares with Tom. She is seen as an "invalid" or "challenged" type of character. Yet, the play is actually about her own sense of "newness," something that is shown when she has to negotiate Tom coming back home after a trying fight with Amanda, and even when Laura is shown to be more realistic and more emotionally centered than Jim O'Connor. The ending of her blowing out her own birthday candles represents a sort of psychological regeneration within her, a new form or future within her own sense of being in the world.
For Miller's work, this element of regeneration is central to the development of Willy Loman. Willy struggles with who he is in the world, and seeks to create a new state of being for himself and his family in a world that is slowly phasing him out. He struggles with this throughout the play. His desire to "hit it big" is nothing more than a materialist version of regeneration, as he believes that a new life and future are intrinsic to material success. His desire to "be something" in financial terms is the "magic bullet" for him to regenerate into something new, or at the very least to not be what he is now. In this, Willy represents the same theme as Laura, but without the positive ending. Willy can experience the power of regeneration through death, indicating that death, the end of life, is the only path for him to bring newness into the world, even though he is no longer a part of it.