What main ideas serve as major headings in your outline?
This is a good question. If you a writing an outline for the purposes of study or writing an outline for a paper, the major headings are important. Let me address this question in two ways: an outline for study and then an outline for a paper.
First, if you are writing an outline for the purposes of study, then the major headings are the major points of what you are studying. Under the main heading, you should have points that support your main heading. So, if you are studying the Fall of the Roman Republic, you can create the following headings: (1) Political Fighting, (2) Political Corruption, (3) Civil War, etc.
If you are writing a paper, then your main heading will be the main supports for your thesis. So, if your thesis is: "War is never justified," then your main points would be why you think this thesis is supportable.