The range of acceptable topics for a senior thesis is as wide as the discipline, but most senior theses will fall into one of three categories.
The first is the theory paper which presents some aspect of the world through the lens of a model. The goal of the theory paper is to improve the conceptual underpinnings of the analytical tools that economists use to understand the world and make predictions about it. It can introduce an entirely new model, or critique and extend an existing model.
The second is the empirical paper which enables us to test theories and measure actual relationships in the data. The goal is to use statistical methods to uncover and explain causal links and evaluate policies. It applies existing econometric tools and approaches to new or updated datasets and topical policy questions, or improves upon existing methods.
The third possibility is a combination of the two. In this case, an empirical fact might motivate turning to a theoretical model to rationalize it, or a theoretical model is used to derive hypotheses that are then empirically tested.
General guidelines for Economics Senior Thesis 2019
Writing tips for economics research papers (Nikolov, 2013)
How to write a literature review (Romem, 2012)
Looking for a data set? Check out google!
Tips for Development studies from the World Bank. Development Impact: News, methods, and insights about impact evaluation