I think that you can go into a variety of domains with a thesis statement on the drama. A very relevant thesis statement would discuss the balance between local and external colliding realities. For example, the Hillsboro predilection for spirituality and the belief that Darwin was wrong collides with the scientific principle of discussion and with freedom of thought. This might be a very interesting topic to explore in terms of how American History has navigated its path when local interests run counter to national or external belief systems. This has been seen in our nation's history with issues of artistic expression and Civil Rights.
Another interesting thesis statement to bring out would be the role of religion in American History. The dominant force of religion in Hillsboro, led by Reverend Brown, is one that seeks to gain power from silencing dissent. It might be interesting to explore how this pattern has also been a part of American History. Whether one wants to pull from the Salem Witch Trials or the rise of the Religious Right in the 1980s, the role of religion has played a vital role in the development of American culture.
Given how the trial and the fundamentalism surrounding it emerged in the 1920s, a period known for its liberal moral stance and permissive social attitudes, it might be real interesting to explore how religion operates in such a dynamic. Finally, I think that it might be interesting to explore how Darwinian thought is viewed today. In the modern setting, Darwinian thought is viewed with some degree of the same skepticism with which it was viewed in the time period of the drama. The emergence of intelligent design as well as school boards that demand guidelines be followed on how human origin is taught is reflective of the same tension that is seen in the play. These might be possible areas upon which to focus regarding your thesis statement.
What originally started as a publicity stunt for a small town quickly grew into a political, moral, and religious hotbed of activity. Did the city council benefit from bringing the trial to the town as much as they had hoped for or did it hurt the town in the end?