The first step is to explore systematic theology in your dissertation. This concept involves taking all of the Bible's topics one by one, such as God, sin, greed, and others, and approaching each as a singular unit. For instance, you might research all the references of greed in the Bible and make note of a few conclusions about the impact of greed on mankind throughout the progression of time. This will give you a well-rounded understanding of what theologians and philosophers had to say about that particular sin. You also could take a broader concept, like sin, and trace patterns throughout the Bible.
A dissertation in Christian doctrine would explore the teachings of Jesus Christ, as well as the foundation of his heritage and the implications of his deity. For instance, your dissertation could explore the life of Christ and how it impacted his ministry in later years or the teachings of Christ and how they connect together. You may also consider exploring Christ's teachings and explore how those teachings would be received within modern-day society. Another avenue is to delve into the philosophical teachings and implications of the Trinity: Father, Son and Holy Spirit.
The philosophy of religion in your dissertation is another avenue you might think about pursuing. This area goes beyond the teachings of the Bible to explore the religion as a whole and why and how it exists. This questions the existence of God and explores philosophical questions and answers throughout time about the organization of religion and the possibility of an all-powerful, omniscient being.
Religious ethics and how they are addressed in today's society would be another appropriate theology dissertation. This topic explores moral problems that plague society and how certain religious beliefs can help humans find a solution to these dilemmas. Another avenue would be to compare the religious ethics across all denominations or throughout the various religious doctrines. You could also compare the ethics of non-Christian religions with those of the Christian faith; for instance, you could compare practices and ethics of Christians with those of Muslims or Buddhists.