How about asking the question, whether or not the American Civil War was really about the emancipation of the slaves. Then you could go into details about the motives each side had for going to war.
Or:
Was it John Brown's actions at Harper's Ferry that sparked off the civil war or was it waiting to happen.
Or:
Take a feminist perspective on the civil war and discuss the role of women in America during that period. You could pick women from different backgrounds, ie. one from the north, one from the south or a black slave. Compare and contrast. There is plenty of material - letters, memoirs etc that you could use.
Or:
The role of black men as combatants on both sides. Think of the film "Glory".
Or:
Conditions of POWs in the notorious concentration camps, such as Andersonville.