Dissertations in theology can be much more varied than biblical studies or tracing the effects of Vatican II on specific groups or issues. For some, the study of the poetry, music and art associated with the Bible may be rewarding. For example, you could research a specific art form, such as clay pottery or its related imagery. However, others may wish to be less conventional, and fortunately, theology is an interdisciplinary field that lends itself to fascinating research.
Since theology, psychology and philosophy are closely related, it would make sense that many good dissertation topics would be in those areas. For example, you could study the role of emotions with respect to well-being and emotion-focused therapy (EFT), which Psychiatry Online defines as therapy where "emotion is fundamental to the construction of the self and is a key determinant of self-organization." Your research could be to develop a Christian counseling or psychological care model in that area. Other topics along these lines would include an examination of Christian psychology when faced with the Holocaust, or you could look into the philosophical angles on thought and existence, with the intent of theorizing the relationship between religion, theology and science.
If you prefer a less-philosophical dissertation topic, you could look at the service side of theology, especially that associated with religions like Christianity and Islam, both of which stress helping the more unfortunate. For example, you could look at the idea of hospitality as a spiritual or religious practice, perhaps even a mandate. Such research would take into account theology, practice, philosophy and ethics. Other types of research along these lines could include a historical survey of relief to the poor, perhaps one concentrating on a specific era or geographic area, or a study of descriptions of God in various cultures and its relationship to the concept of hospitality.
Theology and literature are often studied together, so you'll find almost no end to the types of literary topics that lend themselves to theology dissertations. You could study literature of a specific century or writer to examine the relationship between realism, fantasy and epistemology. Such research could examine authors like Charles Dickens, the Brontë sisters, or Oscar Wilde. Other topics in literature would include studies of the prolific John Henry Newman's treatises on Christianity, or John Milton's theology in "Paradise Lost," or George Herbert's spiritual journey in poetry, or John Donne's juxtaposition of religion and love in his poems. With any literary topic, you could look at angles such as theology, community, hermeneutics or devotional practice.
Theology dissertations do not have to be academic exercise if you don't want them to be. As a discipline, theology lends itself to sociological and political stances that can have real-time effects in communities or worldwide. Topics along these lines could lead to your examining the Western mind and its reactions to genocide, particularly when examining the self-identified religious layperson. Other topics could include social responsibility, engagement with spirituality, outreach to ethnic minorities, reactions to racial discrimination and feminist support in various sects of Christianity.