What is a thesis about Elizabeth Cady Stanton and Sojourner Truth's work for women's rights?
The most interesting possibility for a paper topic would involve the contemporary idea of "intersectionality."
Intersectionality is a concept developed by the legal scholar Kimberle Crenshaw to explain the ways in which the nature of discrimination changes, not based solely on one demographic attribute, but rather on several. For example, white women have experienced discrimination in different ways than black women. White women are also capable of exercising "white privilege" (see: Peggy McIntosh) while also experiencing gender discrimination.
Perhaps you could pursue an answer to the following question: What are the ways in which both the women's rights and abolitionist movements neglected black women? Consider Sojourner Truth's famous speech, "Ain't I a Woman?" in your response.
You could also take on a less critical outlook and analyze the ways in which the abolitionist and suffrage movements worked together and reinforced one another. Frederick Douglass was a key figure in the fights for both abolition and suffrage. He argued that he could not fight for his right to vote as a black man while neglecting the right of women to vote. Thomas Wentworth Higginson, a former Civil War officer and Emily Dickinson's earliest reader, was another fighter for suffrage.