The critical literature on the theme of “madness” in African diaspora literature continues to expand. Researching and writing a paper for a master’s degree will benefit from first narrowing down the topic. African diaspora literature includes a range of literary genres, including personal narrative, fiction, and poetry. Writers of African heritage have been recording their experiences away from the continent for centuries. One factor to consider is whether you plan to consider only literature in English or also that in other languages and published in translation.
The concept of “diaspora” generally applies to any location beyond the continent, so it might be useful to narrow down the geographic scope of the authors or their subject matter. Will you include works written only by African writers living in other places? Or will your scope encompass their descendants born in diaspora? For example, a paper that considers “mad” female characters in twentieth-century Afro-Caribbean fiction would probably raise different questions than one that addresses madness among enslaved women in English colonies.
For thematic concerns, salient issues include the early establishment and continuing association of madness with religion, which may be labeled—especially for women—as “witchcraft” or “voodoo.” In more recent works that address the subject as a medical and mental health issue, you could address how the consequences are depicted, such as tragic death or criminal conduct.
Recent explorations of the topic that you might find helpful are Disturbers of the Peace: Representations of Madness in Anglophone Caribbean Literature by Kelly Baker Josephs (University of Virginia Press, 2013) and Black Madness :: Mad Blackness by Therí A. Pickens (Duke University Press, 2019).