Some additional thoughts that have occurred to me:
1) Following up on mukk_in's ideas: Same-sex marriage (and related practices, like ordination of gay people) is considered acceptable by many Christians in North America and Europe, but sinful by other Christians in North America/Europe, and also considered sinful by many Christians in the Global South. For those who consider it sinful, are chains of reasoning used in NA/Europe the same as the chains of reasoning used in the Global South, or are they different? For those who consider it acceptable, what different chains of reasoning do they use? How much of this reasoning is valid, and how much is not?
2) The transgender phenomenon is getting wide public attention for (I think) the first time in Christian history. This means you might get to be one of the first theologians to consider it in a systematic way. Christian communities seem to be jumping rather quickly to either "Ick, that's sinful" or "Of course it's not sinful", but I haven't yet seen it addressed in a careful way in volumes of theology. Questions would include: Is it morally acceptable for people with gender dysphoria to have gender reassignment surgery? (or, is it a positive good? or, is it sinful?) Does God assign us a specific gender, and if so, how do we figure out what that gender is? Or is gender a superficial quality, like what color my hair is or how tall I am? Or does it vary from one person to another? This topic might fit well with your interest in the doctrine of the person and God (who is beyond gender) creating us as sexual/gendered beings.