You may visit https://shodhganga.inflibnet.ac.in/ on which number of Ph.D. theses are uploaded. In search field just type keywords on capital structure, you may find the list of theses, which are downloadable also.
You can conduct research on the impact of cultural factors on the capital structure of multinational companies. the aim of the study is to investigate how cultural factors influence the capital structure decisions of multinational companies or address the impact of political and economic risks in host countries on the capital structure decisions of multinational companies operating in those markets
If you are interested in Inflation Accounting, it would be a good subject. Capital structure is affected by the inflation in some foreign countries like, in Venezuela, Argentina, Turkey. Inflation sweeps the profits of the companies and makes them still pays taxes if their financial statements are not adjusted for inflation. Accordingly their capital structure will be deteriorated because of this. Their Equity will shrink and debt equity ration will go to south.