Your paper should be 8-10 pages single or 1.5 spaced. Longer papers are also OK. There should be a short abstract, introduction where you explain what the topic is and the main questions to be addressed, the main analysis where you carry out the analysis, and conclusions or discussion (where you give your own opinion on the subject and for example the current state of affairs following the antitrust case if that is your topic). The main analysis may be split to more than one sections if you feel it is appropriate. For example one could be theoretical analysis and the other empirical analysis, or if you are writing about an antitrust case, economic theory and legal case. You should have two or more references. One reference could be the DOJ site. Online or even Wikipedia references are OK for recent developments. You can reference the textbook as well. One of your references should be a published research paper.
For your topic, choose a firm or an industry that was subject to antitrust or regulation and write about it. For example you may expand on the topic of your class presentation. Please note that the presentation papers may be somewhat dated and a good paper would supplement that analysis with more on what has happened since the case. For example when it comes to NCAA in addition to the case in the paper there is also NCAA vs Alston and its aftermath. Another example, you may write about whether Amazon should be broken up and the current antitrust cases against it in the US and Europe. But that is more difficult subject.