One good idea for a doctoral thesis is to explore issues that low-income families face when it comes to public health. You can do a dissertation on specifically immigrant families in low-income communities and talk about how the language barrier, government policies and social stigmas all play a role in the lack of access to quality health care for them.
For those who are interested in exploring religious trends in America, you can discuss specifically the religious trends among minorities in the country. You could narrow in three important minority groups; African-Americans, Asian-Americans and Hispanic-Americans. Particularly, interview minorities in the 18 to 35 age group, and examine whether there is a generation gap between them and their relatives regarding the importance of religion.
In light of the increasing globalization of the economy, American youth will require proper training for a competitive workforce that will require skills that weren't necessary 20 years ago. You can write a thesis that asks whether today's education system is failing or adequately preparing them for this type of job market. You could interview high school principals, education officials, teachers, parents and business owners to get an idea of what's wrong with the education system and what solutions will work to alleviate the problems.