I love this novel in every way, and there are certainly some more "literary" themes to be found in this work, so take my idea as perhaps a bigger-picture or over-arching theme. It seems to me a major theme in A Tale of Two Cities is illustrated in the first lines of the novel--"it was the best of times, it was the worst times." This novel showcases the best of who we are (Sydney Carton) and the worst of who we are (St. Evremonde).
It might be expressed this way: In the middle of great trials, adversity, and evil can be found great victory, perseverance and love. To support this thesis I'd use the characters and incidents which best represent those horrible and wonderful aspects of life. For example, I'd use Monsieur the Marquis as the epitome of evil, and we have plenty of evidence (quotes) you can use. In contrast, I'd use either Sidney's avowal of pure love to Lucie or something from his sacrificial switch with Charles to show the greatest love. Any such character traits or characters would fulfill your requirement, I think.
Hope this helps. If you're looking for something, as I said, more "literary," I've included a link to themes from this novel below. I've also included a link to characters from the novel, in case you need a little help there.
Victorian writer Charles Dickens used his contemporary's, Thomas Carlyle's The French Revolution: A History as his primary source for his novel, A Tale of Two Cities. In so doing, Dickens adopted the philosophy of Carlyle that chaotic events demanded "heroes" to take control over competing forces erupting within society. Without denying the importance of practical and economic explanations, Carlyle perceived these forces as "spiritual"--the hopes and aspirations of people that took the form of ideas.
One of the ideas that took form for Dickens after reading the work of Carlysle is that of resurrection, the recalling to life of the "heroes" needed to compete against the forces of the French peasants. Two such heroes are Dr. Manette and the unlikely dissipater, Sydney Carton. For these two men, the French revolution acts the force which gives rise to their heroic natures. On the one hand, Dr. Manette, who has been "recalled to life," returns to France as a hero who has been released from imprisonment in the Bastille to effect the release of his son-in-law, Charles Darnay. And, on the other hand, the wayward Sydney Carton finds direction to his life in his love for Lucy with the Revolution. Carton rises as a hero, the sacrificial victim in place of Charles Darnay, who is then able to return to his wife Lucie. He is Charles Darnay's resurrection: "I am the resurrection and the life." In Carton's death, his life is resurrected as it gains value, a value it did not hitherto possess. Indeed, Carton is the spiritual form Carlyle envisioned in the hero who would combat such chaotic times as the French Revolution: As he contemplates his self-sacrifice, Carton recalls the words he recalls having heard at a funeral with his mother:
...he that believeth in me, though he were dead, yet shall he live: And whosoever liveth and believeth in me shall never die.
Even humorously, with the "resurrection man" Jerry Cruncher who digs up graves for profit, the theme of Resurrection courses through the narrative of A Tale of Two Cities until the spiritual heroes, Dr. Manette and Sydney Carton, arise to defeat the chaotic events of the Revolution in the persons of Madame Defarge and John Basard, and the Vengeance. Indeed, it is a wonderful tale of two cities and the heroes from each that are resurrected.
RECALLED TO LIFE