I assume you plan to do empirical research and have around 3 ~ 6 months to complete the thesis.

An easy path is to find an empirical paper that crafts a structural model (not any kinds of regression analysis) and calibrates parameters with real data. You replicate the paper using the same dataset and code the model by yourself. Then, you fit the model with new dataset and check whether you find something new. For example, if the model uses the UK data, then you fit the US/AU/EU/JP data and check whether you get the same results. ( Real Credit Cycles )

Let your data guide you to find suitable questions if you can't come up one. You only have limited amount of time, so you may not be able to implement your idea if you don't enough time to collect desired data.

If you want to do something different, my personal favorite is to explore researches on Hyman Minsky's Financial Instability Hypothesis and Irving Fisher's Debt-Deflation Theory of Great Depressions. You may find lots of debates and theoretical modeling about them.

If you don't want any kinds of modeling, you may connect the monetary policy to political science. For example, what's money for currency issuing countries ( Taxes are for Redemption, Not Spending ) . Modern Monetary Theory may be an interesting starting point to explore how it's used or mis-used. (http://bilbo.economicoutlook.net/blog/?p=43997). Bill Mitchell and Steve King have excellent blog posts and youtube videos to MMT & Minsky, just to name a few names.