There certainly are other examples of symbolism in this play. One very important example is that of seeds. If we think about seeds, they do symbolise potential, growth, and future promise. If you read the play and identify the various references that there are to seeds and to Willy gardening in his yard, you can see how this is related to the American Dream and Willy's hopes of eventual success, wealth and prosperity. By far the most poignant example of this is after Willy "comes to" in the restaurant when Happy and Biff have abandoned him and asks Stanley, the waiter, if where he can buy any seeds. Notice what he says to him:
I've got to get some seeds, right away. Nothing's planted. I don't have a thing in the ground.
This is of course after he realises that his hopes for Biff's great future have come to nothing and he has no chance of being famous or gaining the success that Willy hoped he would. This is why in the final scene he plants seeds in the darkness of his home in a deranged kind of way. Seeds to Willy symbolise hope in the American Dream and its potential to give him and his son the success they have always desired.