There can myriad choices for a dissertation topic pertaining to Finance. Some of the prominent ones are as below:
Private equity (Here, once can research on PE investments in some of the major economies and the quantum of returns these investments have generated over a period of time and then compare top economies and back with the reasons behind growth of PE in those economies)
Equity markets (One can take the case of the emerging markets like China, India, Singapore, etc and formulate a hypothesis with regard to the returns that can be generated in these markets and compare them with returns generated)
Subprime crisis and its aftermaths (Changes that the crisis has brought about. These changes can accounting changes, or lending rules becoming stringent)
Financial Leverage (Some of the most popular ways of raising money and their effects. Which methods are the best for what kind of companies, and during what stage of the cycle)
Merchant Banking (The new trends and opportunities in the business and how it has come along).
There are several more topics on Finance, but the most appropriate thing is to take up a topic one is really interested in exploring. Many companies offer help with dissertations. I do not suggest to outsource your paper and submit it as your own, but one can definitely approach such companies and use their research for ideas and then submit their own paper. I personally approached quite a few companies for my dissertation, but found only a few to be legitimate.
Email me for any help or you can try mailing [email protected] as these guys also helped me with my dissertation.