This sounds like a great paper. If you are writing a paper on how the United States progressed in terms of power, freedom and equality, then you will need to narrow things down. As you can probably imagine, the topic is enormous. Let me suggest a few possible thesis statements.
First, one of the most obvious ways in which American has grown in equality can be seen in the presidency of Barak Obama. This is a historic time in America. The dreams of Martin Luther King Jr. for example has been actualized to some degree. This can be an excellent topic for a paper. You can also tie in the theme of freedom.
Second, another possible thesis can be on the growth of American power in terms of the economy. While it is true that we are going through a difficult time in our economy now, we are still the strongest, largest, and most important economy.