Choosing a topic is a critical part of writing your seminar paper. Significant thought and research should go into your topic selection to avoid problems as you continue your research process. If you do not spend the time to ensure that your topic is important enough to you that you can write about it for thirty to forty pages, your interest will wane and you will be unable to write an engaging paper. Additionally, you must do a significant review of the existing literature to ensure that your topic is original (this process is sometimes called preemption checking). Think of the body of scholarly writing as an ongoing conversation. You want your contribution, in the form of your seminar paper, to add to this conversation, bringing new ideas or inviting scholars to see something in a new way. It will not be possible to add to the conversation, or to avoid merely repeating what has already been said, if you do not first read the existing literature. One approach is to begin with a general topic that interests you and to use your review of the literature on that issue to refine your topic into a unique thesis. Only if you find yourself unable to come up with something original to say on your topic must you discard it completely.
Your topic should be novel, non-obvious, useful & sound.
(Taken from Jessica Lynn Wherry & Kristen E. Murray, Scholarly Writing: Ideas, Examples, and Execution KF 250. C625 2019.)
An important characteristic of a good topic is that there is an ongoing debate on the issue. Circuit splits, or disagreements between appellate courts' positions on the legal issue, can be a rich area from which to mine topic ideas. Writing a paper on a circuit split issue provides a topic that is both controversial and is likely to have ample sources to cite.
Google: Your topic+ legal research guide
Including guides on how to pick a topic.