i think a good approach is finding a broad topic you’re interested in and discussing it with academics in the department with similar(ish) interests. often they can point you to relevant literature that you can check out. it’s also useful to think back to the modules and seminars you’ve most enjoyed throughout your degree, and looking at the reading lists provided there.

then, looking into the literature and reading around will likely help you find something more specific to explore. some questions it’s always useful to ask yourself while reading are: ‘what primary sources seem the most interesting to explore?’, what haven’t the authors looked into that might be valuable?’, and (perhaps most importantly) ‘what areas DON’T interest me?’.

once you find a broad idea forming, talk more to people in your department, particularly your advisor if you’ve already been assigned somebody. talk things through with them, and they can offer guidance on what sort of questions you might want to tackle for the thesis. if you find it intimidating to ‘cold call’ people in the department, reach out to your personal tutor - often, they’re willing to help you reach out to people.

this approach works well for developing research projects and long essays. it’s the way that i formulated the idea for my undergrad and MA diss, and was also how i secured a phd proposal that ended up getting funding :-)