I'd add that a lot depends on the requirements of the paper. If you are expected to do original research, the biggest single issue you face - bigger than the choice of topic in fact - is choosing a subject that is in quite an unusual "sweet spot". It needs to be something that has not been exhaustively researched, but which you can do archive research on in a very limited period (in the UK, at least, it's typical for undergrads to spend at best 4 or 5 weeks on the research phase of their dissertation) - and so you have to be absolutely certain you will be able to uncover useful material in that period in whatever archive you are using. Very often, moreover, you won't have the chance to do research in multiple archives, which further limits your options.

For these reasons I often recommend students who have no compelling topic they are desperate to research to choose a subject that can be researched via digital archives. For example, the way in which some significant incident was covered in the press. Newspapers count as "primary sources" and modern digital archives such as the British Newspaper Archive can be accessed from your desk without the need to spend long periods physically working in an archive – and with the guarantee that the advanced search facilities available nowadays will tell you almost immediately if the resource has enough material to allow you to complete your paper.

I do think you miss something by not experiencing what real archive work with paper-based sources involves, but if you're not planning to pursue graduate research in history and just want to find a "do-able" topic quickly, the digital archive route is well worth considering.

I would love to help you but I think it absolutely has to come from within you, otherwise you won't have the drive to see it through. I feel bad for not giving concrete advice, but I don't think you can 'outsource' something like this :/

I originally wanted to look at advertising in 1920s America, but I had a lecture on English deer parks and had to change because I knew I'd never tire of it. I discussed it with the lecturer, and he gave me general reading on the topic, eventually narrowing it down by time period, then theme (knew it would be Tudor/early Modern, and considered the types of people that owned park, parks as status symbols, their popularity, their decline). Then, in one article I found a piece of historiography that had gone unquestioned since 1979, which I fundamentally disagreed with. So, I explored that debate for my dissertation, 10,000 words. So, get recommendations from a professor. Make a list of potential themes, then the established historiography, then find your niche. If you want more help, pm me. It's immensely enjoyable, but getting the topic right is crucial.