Awww, this is a difficult question. You should first identify a specific issue you want to analyse or an object you want to create in the area of Capital Structure. The request is too wide. I may suggest that you do a comparative case study of capital structure in organizations across some sectors of the economy - the nuances and, since you are talking of PhD thesis, you should think of coming out with a model, perhaps determine e.g. the ideal capital structure appropriate for all sectors that that type of economy.
Based on my previous work on deterioration of paid up capital-equity in the face of inflation and devaluation might be an idea. Financial statements prepared in local currencies and pay their taxes with local currency. However, inflation and devaluation in the local economy distorts the financial statements. Companies still pay taxes despite the fact their equity lessens on the face of inflation adjusted and/or hard currency converted financial statements.
You can study on Financing Decisions of Capital Structure considering Leverage and EBIT analysis, or/and study the impact of capital structure on firm value using any one of the approaches like operarating income approach, net operating income approach, and MM approach of capital structure decisions. Where you can take variables like value of the firm as dependent variable; and cost of capital and market value of shares as independent variables.