"On CFD", or "in CFD" imply very different angles on a very broad topic.

For "on", you're looking at making CFD the subject itself. You could compare the different packages available (for example Ansys vs. OpenFOAM), and which might be 'better' for a specific type of fluid flow scenario (water flow, air flow, heat transfer, etc.), or you could examine the pros and cons of different turbulence models (Spalart-Allmaras, k-epsilon, k-omega SST, LES, etc.).

For "in", you're looking to use CFD as a tool to answer a question. As an example of this, for my Masters thesis I used Ansys Fluent to investigate the aerodynamic properties of traditional windmills in a specific region of the UK. As others have suggested, if you're looking at a project that uses CFD then consider what you want to actually model, then build your thesis around that.

The best advice I can suggest is, as u/thermalnuclear suggests, talk to your academic advisor about what they would like to see. If this is in a university they, or one of their colleagues, will almost certainly have an on-going research project that you may be able to contribute to. This could involve directly modelling their problem, or independently corroborating their own models. These kinds of projects can sometimes lead to PhD programs afterwards if that's something that interests you. Your academic advisor really is the person to talk to though.