I would suggest starting with the W's: who, what, where, when, why, and how?
For "who", you could research who is banking. Are homeowners taking out more or fewer loans? What about students? Who is using microfinance in the developing world? Similarly, you could focus on a particular bank or CEO.
For "what", topics are also easy to think of. What products are banks selling more or less of? What world events or trends affect banking (e.g. the fall in oil and commodities prices, the Federal Reserve's tightening of monetary policy while the ECB loosens it, or the effects of the sanctions lifted from Iran)?
For "where", just pick a place. Particularly interesting are the banking climates in China, Japan, Europe, the USA, Iran, and the Asia Development Bank.
"When" questions are a little more difficult, but you can still find several. When will the Federal Reserve raise interest rates? What times of the day/week/month/year see the most consumer or commercial banking activity?
"Why" and "how" are often connected. Why are Greece and Britain considering an exit from the Eurozone, and how would that effect banking in Europe? Why is inflation still low in so many places, and how does that affect the demand for loans and savings accounts?
The bottom line is that you should choose a topic that interests you. My suggestions indicate that I focus on international issues and monetary policy. Even if your interests differ, this should give you a means to choose a topic. Consider what you want to write about and start asking yourself these kinds of questions. A topic should emerge quickly.