There are lots of different ways you could go with this but one thing you might try is something about fate. Fate was very important to the ancient Greeks. They believed that a person's destiny was written in stone. If a human being tried to deny and defy his fate, as Oedipus does, a greater tragedy would ensue. When a mere mortal tried to write his own fate, he's elevating himself to the level of the gods. This is an example of hubris and won't be tolerated by the Olympians. Maybe your thesis question coud be: "What is the role and importance of fate in Oedipus Rex and Burial at Thebes?"