Since illustration involves the use of specific examples to support a general point, you will need to consider how you will provide support for the American symbols that you choose. In writing your thesis, of course, you need a controlling idea which you can develop. For instance, you can illustrate how the meaning of certain American symbols have changed. (Symbols, of course, by definition only have a meaning that people provide them.)
One symbol whose meaning has changed in America is the peace symbol. Whereas it once meant an antiwar symbol, specifically against nuclear war, it was worn by those of the counterculture, but, in recent times it now represents the positive desires for love, hope, and peace. Of course, the American flag has assumed different meanings to people in the United States. The flag-burning of the sixties signified the hatred for the federal government in its declaration of policies, specifically the Vietnam War. The stars and stripes are rejected by some in the South, as well, who feel the federal government has unjustly altered the way of life in this area. For this reason, and for others, the battle flag of the Confedercy is flown, instead.
Hi I don't know what you have so far, but how about the following: Symbols, those used both covertly and commonly, often provide a stimulus for learning and performing in different arenas.
Let me know what you think. I can also expand upon this...