When selecting a topic for your research prospectus, you want to find a middle ground between questions which have already been thoroughly addressed and questions which are not answerable. Try not to choose ground that has already been comprehensively covered, but try also not to choose a question which you cannot reasonably answer/argue.
You want to select a topic that holds interest for you, and that offers some room for new research, but you must also select something which is feasible.
For the purposes of this class project, feasible will mean two things:
Your question needs to have a relevant, existing primary sourcebase so that it can be investigated. But this sourcebase needs to be one which you can reasonably explore in this class, either through published sources, digital archives, or well-developed published/online archival finding aids.
Some questions may, in general, be possible to answer, if you were able to personally spend time in some obscure archives which do not have digitized materials or online finding aids--however, such a question would not realistically be manageable when working as a student in this class.
Robert C. Williams describes feasible research topics like this: "there should be sufficient primary and secondary sources on the topic, but not so many that you could not possibly get through them in the time allotted" (52).
You should see how this statement ties in with comments on the previous tab, regarding the formulation of a research question that is not too broad but also not too narrow.
Williams, Robert C. The Historian's Toolbox: A Student's Guide to the Theory and Craft of History. Second ed. Armonk, NY: M. E. Sharpe, 2007.
Take a look at the next several few in this guide for more details on assessing feasibility by considering the secondary and primary sourcebase.