There are several options; Sula lends itself directly to feminist criticism. Sula's family is often criticized by other residents of the Bottom, and particularly the women of the family are seen as less than respectable. Nel, on the other hand, comes from a socially upright family. Viewing Sula from a feminist perspective would allow the reader to analyze the nature of gender roles that is apparent in the novel. Further, the characters' challenging of these roles would be of issue as the reader attempts to view the motivation behind much of Sula's illicit behavior.
I would say Gender Roles and the effect of Race is very important. Hannah slept with all the men which led others to not view her as well and Sula ended up bringing up a bunch of Robins with her which seemed as if she was evil. On top of that, they had feminist views which led some in the Bottom to be unhappy that they did not conform to gender roles. The people of the Bottom view Sula to be bad when they hear she has slept with a white man.
Toni Morrison was inspired by the magic realist writers from South America (Gabriel Garcia Marquez in particular) and wanted to create a novel that addressed African American themes more thoroughly through this literary form. The magic realists believed that by addressing serious and often surreal problems of brtuality and violence under dictatorships could only be fully expressed through the absurd. By normalizing the absurd (the magic realist aspects of their novels are rarely treated as extraordinary), they were able to reveal the true horror of the circumstances under which they were forced to live. Morrison does the same with Sula in addressing the racial climate of the WWI and WII eras for Black Americans. A critical analysis of magic realism and the ways in which Morrison employs them in Sula could be an option for you.