The Civil War is popularly known as the war that ended slavery. The tensions between the two factions in the United States go back to the forming of the nation. A research topic for a Civil War paper could focus on these conflicts, such as the rights of the individual states versus the formation of the union of the states. The history and abolition of slavery in some states and in other countries is a factor that lead up to the Civil War as well. Researching historical points of view the causes will result in a full paper.
Historians have written a great deal about the strategies of generals in battle. There is a wealth of material, including historical documents, showing exactly what the strategies were in the war. For a paper topic, pick one big battle and research how it was planned, how the plan was executed and how the outcome of the battle impacted the war. You will be sure to find several books and videos for your research on this topic.
Even though the Civil War was a domestic war, the relationships between the Confederacy and foreign nations and those same nations with the United States, or Union, is a topic with plenty of material. The most prominent foreign nations to research are England, France and Spain. You can choose one nation and its relationship with either the "Rebels" or the "Yankees" as your topic if you find enough information. You can also write a comparison between the relationships with all three of those nations with both the Confederacy and the Union.
In the century and a half since the end of the Civil War, plenty of historians have proposed theories of what might have happened if the Confederacy had been victorious over the Union, or if the Union had simply not fought against the Confederacy. You can research the historical theories of what North America might have been like if there had been both the Confederate States of America and the United States of America, along with Mexico and Canada. You can also research the theories of what North America could have been like if the Confederacy had prevailed and captured Washington, D.C.