Some grad students do original research and some do what an old professor of mine called a "literature review on steroids," meaning a thesis that is solely an extensive review of the literature. Either way, there are many interesting topics to pursue in human resources.
One of the most fascinating questions in human resources is what motivates people. Is it simply a question of rewards and punishments? Or is it a question of autonomy and interesting work? There is a great body of literature on the subject of employee motivation to explore, as well as the possibility of creating your own hypothesis and research questions. Some well designed questionnaires given to the employees at a local company might provide some answers, at least on a small scale.
Another topic of interest in human resources is that of training and development, something that HR people are frequently responsible for. Is cross-training effective? Is there any correlation between training and upward mobility in an organization? Is it better to outsource training? These are some issues that could be explored.
Recruitment and employee selection are subjects ripe for research, too. How have things changed for recruiting practices in today's on-line environment? Is it better to promote from within or to seek management from outside? How can one select employees without running afoul of the anti-discrimination laws and regulations? Are personality tests of any use in employee selection?
In each of these areas and many more, there are great areas to do research in and the possibility of conducting your own original research. HR is interesting because it's really about people.