The fall of the Soviet Union is a massive topic and you might do better to pick something more specific within that general region. I'm not sure what this could be, I didn't do modern history, but something along the lines of "the role of Gorbachev in the fall of the Soviet Union" or "the extent to which glasnost and perestroika contributed to the breakup of the Soviet Union", etc. Alternatively, if there is a particularly well-known work about it, you might consider a critical analysis of that work, comparing it with the opinions of other historians. Someone in my class did that for Gibbon's 'Decline and Fall of the Roman Empire', and it worked pretty well because he didn't have to research such a huge topic himself. However, the fall of the Soviet Union is pretty recent so you may not find that views about why it fell have changed much since then. Above all you need to pick a topic that is manageable in size and has opposing historical views. Your question will fall into place later on.
