Here is your rule of thumb for sources and essay length: if you have an eight-page paper, have eight different sources in your bibliography. Ideally, you should have a dozen sources, since you will want to include your Marsilius course texts as part of the bibliography.
First, take a look at the "Suggestions for further reading" bibliography, after the Introduction to the new translation of Defensor Pacis (pp.xxxii–xxxvi). That bibliography will be your best resource, no matter what your chosen essay topic will be.
Next, be sure to read these tips on how to write a good essay.
Finally, here are some recommended resources for possible essay topics:
Stephen A. McKnight, "Understanding Modernity: A Reappraisal of the Gnostic Element", The Intercollegiate Review (Spring 1979): 107–117.
Stephen A. McKnight, "Medieval Order and Disorder in Voegelin’s History of Political Ideas", Political Science Reviewer 29.1 (Fall 2000): 160–191.
Jeremiah H. Russell, review of Eric Voegelin, Science, Politics, and Gnosticism (Wilmington, DE: ISI Books, 2004), in Political Theology 8.2 (2007): 251–253.
Leo Strauss, "Marsilius of Padua", in Leo Strauss and Joseph Cropsey (eds.), History of Political Philosophy (Chicago: University of Chicago Press, 1987), 227–246.
Eric Voegelin, "Marsilius of Padua", in The Later Middle Ages: History of Political Ideas: Collected Works of Eric Voegelin, Vol. 21 (Columbia: University of Missouri Press, 1998), 83–102.
Eric Voegelin, "Science, Politics, and Gnosticism", in Modernity Without Restraint: Collected Works of Eric Voegelin, Vol. 5 (Columbia: University of Missouri Press, 1999), 295–313.
Garnett, George, Marsilius of Padua and 'the truth of history' [electronic resource] (Oxford: Oxford University Press, 2006).
John von Heyking, "Aquinas's Mediated Cosmopolitanism and the Impasse of Ancient Political Philosophy", in Lee Trepanier & Khalil M. Habib (eds.), Cosmopolitanism in the Age of Globalization: Citizens without States (Lexington, Kentucky: University Press of Kentucky, 2011), 70–96.
Thomas P.M. Barnett, The Pentagon's New Map (New York, NY: G. P. Putnam's Sons, 2004).
Roger Scruton, The West and the Rest: Globalization and the Terrorist Threat (Wilmington, Delaware: Intercollegiate Studies Institute, 2002).
Dan Berrett, "Intellectual Roots of Wall St. Protest Lie in Academe: Movement's principles arise from scholarship on anarchy", The Chronicle of Higher Education (Oct 16, 2011).
Lee Trepanier, "The recovery of science in Eric Voegelin's thought", in Lee Trepanier (ed.), Technology, Science, and Democracy (Cedar City, Utah: Southern Utah University Press, 2010), 44–54.
Lee Trepanier, "Eric Voegelin on the Law and the True Substantive Order", First Principles (Nov 30, 2009).
Eric Voegelin, "The New Science of Politics", in Modernity Without Restraint: Collected Works of Eric Voegelin, Vol. 5 (Columbia: University of Missouri Press, 1999).
Eric Voegelin, The Nature of the Law: Collected Works of Eric Voegelin, Vol. 27
Harold J. Berman, Law and revolution [electronic resource]: II, The impact of the Protestant Reformations on the Western legal tradition (Cambridge, Mass.; London: Belknap, 2003).
C. S. Morrissey, Review of Rémi Brague, The Law of God in Bryn Mawr Classical Review 2007.09.46.
Benedict XVI, "The Listening Heart: Reflections on the Foundations of Law" (Sep 22, 2011).
Friedrich Hayek, The Road to Serfdom (London; New York: Routledge, 2001). [economic policy]
Eric Voegelin, "The Origins of Totalitarianism", in Eric Voegelin, Published essays, 1953-1965: Collected Works of Eric Voegelin, Vol. 11 (Columbia: University of Missouri Press, 2000), and in Review of Politics 15 (1953): 68–85.
John von Heyking, "Post-9/11 Evocations of Empire in Light of Eric Voegelin’s Political Science", in David Tabachnick and Toivo Koivukoski (eds.), Enduring Empire: Ancient Lessons for Global Politics (Toronto: University of Toronto Press, 2009), 185–214.
Brian Tierney, "Marsilius on Rights", Journal of the History of Ideas 52.1 (Jan-Mar 1991): 3–17.
Charles Taylor, A Secular Age (Cambridge, Mass.: Belknap Press of Harvard University Press, 2007).
Jacques Maritain, "The Pagan Empire and the Power of God", Virginia Quarterly Review 15.2 (Spring 1939): 161–175.
Pius IX, Quanta Cura (1864), and Vatican II, Dignitatis Humanae (1965).
Rémi Brague, "Sin No More", The American Spectator (May 2008): 28–35.
Eric Voegelin, "The Order of Power: Machiavelli", in Renaissance and Reformation: History of Political Ideas: Collected Works of Eric Voegelin, Vol. 22
Niccolò Machiavelli, The Prince (1515).
Jacques Maritain, "The End of Machiavellianism", in The Range of Reason (1948); originally published in Review of Politics (Jan 1942).
Garrett Mattingly, Renaissance Diplomacy (1955).
Henry Kissinger, Diplomacy (1994).
Angelo M. Codevilla, A Student's Guide to International Relations (ISI Books, 2010).
Benedict XVI, "Christianity Has Always Pointed to Reason and Nature" (Oct 21, 2011).
Harold J. Berman, Law and revolution [electronic resource]: II, The impact of the Protestant Reformations on the Western legal tradition (Cambridge, Mass.; London: Belknap, 2003).
Eric Voegelin, "Luther and Calvin", in Renaissance and Reformation: History of Political Ideas: Collected Works of Eric Voegelin, Vol. 22
Eric Voegelin, "The English National Polity" in The Later Middle Ages: History of Political Ideas: Collected Works of Eric Voegelin, Vol. 21
Jeffrey C. Herndon, Eric Voegelin and the Problem of Christian Political Order [electronic resource] (Columbia: University of Missouri Press, 2007). [review]
Nederman, Cary J., Community and consent: the secular political theory of Marsiglio of Padua's Defensor pacis (Lanham, MD: Rowman & Littlefield, 1995).