My undergraduate background is in history and if I were to be hired in an academic library I would like to be the reference librarian or subject liaison for that department. My topic will be to look at recent scholarly communication/publishing trends within the history field (or the larger humanities, depending on what I find). I think that it is interesting that humanities as a whole has been slower to adopt OA and also that monographs remain more popular than journals.
I think focusing on history would be a good choice, if you can find enough material. Talking to scholarly communications officers at a few institutions may help.