A Marxist thesis statement would highlight the way in which socialism is a better way to live than capitalism, so it would call for an end to capitalism and the holding of all resources by a smaller percentage of upscale society. For example, "History is more of a chronology of class struggles than good versus evil. Therefore, Dickens' "A Tale of Two Cities" shows that the working poor is the most badly treated of all classes."
A rich-versus-poor thesis statement would emphasize Dickens' opening line, "It was the best of times, it was the worst of times." Example: "In Dickens' novel 'A Tale of Two Cities,' the opening lines paint a vivid picture of the story to come--a comparison and contrast of two opposing sides of society."
Since a theme of the novel is about the social differences between the rich and the poor, a thesis statement could address the methods adopted by the poor in the novel to fight the the rich, or those who oppress the poor. Example: "Dickens' novel 'A Tale of Two Cities' may be a subtle reminder that, given the chance, the poor will use the same cruel, even murderous tactics that they consider to be characteristic of the rich."
Dickens was a master at demonstrating that no person can truly know all there is to know about another person. Note this quotation: "A wonderful fact to reflect upon, that every human creature is constituted to be that profound secret and mystery to every other." Sample thesis statement: "Charles Dickens emphasizes in this novel that everyone has hidden secrets. Rich or poor, every person wields power over another just by the things that cannot be known by the other."