One important theme is that of adhering to or rebelling against tradition. This is seen in March's preference for the spirituality found in nature over the traditional and, as he thought, stifling spirituality of the Church. In line with this theme, you might analyze March's motive(s) for enlisting when he has family responsibilities.
I would suggest that the fact that March is a religious man would indicate that the theme of religion is likewise important and how it functions in a time of war. You will want to relate that to the inconsistencies of March's character and the ethical dilemmas that he faces, such as trying to hide the realities of war from his wife. But more importantly, what is the role of religion in a time of war? This would be an interesting question to consider through this novel.
Given that John March is a chaplain in the Army during the time of the Civil War, several themes could be discussed through this lens. One is the idea of war itself. You could look at the death, destruction, and stress caused by war paralleled with March's insistence on hiding these realities from his wife (and possibly himself).
As a chaplain, March is a religious man, but he is considered unconventional. You could look also look at this idea of "conventional vs unconventional" in the way that March is the one who brings a sense of peace and hope to the soldiers (esp. when they are hurt or dying), delivers sermons, and is looked to as a man of some wisdom and inner stability, and yet he struggles inwardly with his own sense of lust for a woman other than his wife, and the guilt that these feelings cause.
When March returns to his family, he isnt the same. Before his Civil War expierences he was an idealist, but now he is no longer one. you could always write about a point in his life that makes him a dinamic character