The topic should be broad enough to inspire many points of view but narrow enough to be focused and possible to research in a relatively short period of time. For example, students could debate the question, "Is Human Nature Fundamentally Good or Evil?" This question has been debated for centuries, so abundant written material is available to formulate a supportable point of view. Students could approach the question from numerous standpoints: Enlightenment thinkers, religious sources, historical and political events and more.
Debate is more meaningful and relevant when it's put into the context of the literature students are reading. World War II, for example, is a topic that is both rich with subjects for debate (the morality of firebombing, the mythologizing of war, war and the economy, etc.) and you can tie it to a vast literary tradition: Kurt Vonnegut's "Slaughterhouse Five," John Hersey's "A Bell for Adano" (or Hersey's "Hiroshima"), and Gunter Grass's "The Tin Drum" are just a few examples.
You can make this activity multicultural through literary selection; i.e., by debating issues raised by writers from a wide variety of ethnic and cultural backgrounds. You can also do it by seeking multicultural perspectives on a controversial topic. If you are debating the topic "Were the atomic bombings of Hiroshima and Nagasaki morally justifiable?" the teacher could ask students to approach the question from the viewpoints of an American soldier and a Japanese family in 1945.
Writing an essay on illegal immigration is a different experience from staging a theatrical piece, or debating in front of the class in groups, or doing role-playing. Each of these modalities will give students fresh perspectives on the topic being debated. You might, for example, ask students to role-play a scene between two literary characters. Or you could ask students to write essays about how they think illegal immigration affects their own personal lives.