Original: Your article/note should offer original research, ideas, and/or conclusions; it should bring something new to the world of legal scholarship.
Topical: An article/note usually should concern itself with issues of current interest.
Unlikely to be Rendered Moot in the Near Future: It could be pointless to write on a circuit split if the Supreme Court has granted review and will render a decision before your note will be published. Also, avoid basing your article on volatile world situations that could change substantially at any time.
Narrow: You cannot take on the world. It is better chose a narrow topic and examine it thoroughly, than to be overwhelmed by a broader topic.
By Beatrice Tice, University of Michigan Law Library.
Each of the books listed below has a chapter on choosing a topic: three varied approaches.