Usually when one does research into a topic, certain main ideas tend to pop out. Have you come upon any of these yet? If you are focusing on regionalism, you probably have come across statements that Mark Twain (Samuel Clemens) helped to create and popularize a distinctive form of American literature constructed around American themes and language. He was a master of depicting a colloquial form of speech (as evidenced in The Adventures of Huckleberry Finn and The Adventures of Tom Sawyer), drawn from his experiences growing up in Missouri. Perhaps your thesis statement, then, can focus on the unique ways in which Twain used language to get across his themes. Read the discussion under "style" here on eNotes for some good ideas on how his style affected his themes. Other teachers will no doubt make additional suggestions and you can choose the one that most interests you. Good luck.