Just go with whatever you feel particularly interested in!
If you're stuck with ideas, you could look at the modules that the universities you're thinking about are offering. Lots of themes will overlap & might nudge you in a direction, like a particular playwright, women in Classical society, vase painting etc.
What about classical civ interests you, as said above. If it's the history, pick a period. Also bear in mind what you can get resources for - anything really obscure will make it a lot more work for you!
Women in antiquity, as suggested above, is quite good (if that interests you) as there are quite a few books that are fairly respected (as in, have been suggested for uni reading) that sell well/are in libraries.
I did Classics and History of Art at college, so for my EPQ I combined them and wrote an essay evaluating the accuracy and background of the painting Pollice Verso by Jean Leon Gerome. This definitely helped me focus on one topic (which I'm terrible at doing, I get distracted with interesting facts) but also meant I got a range of information.
As people have already said, you need a subject that you can find plenty of information on. One thing that I would suggest is doing a quick search to find out if there are any talks on subjects that interest you.
A topic like the presentation and treatment of artefacts in archaeology or in museums would give you a lot
to talk about, or you could focus on the development of a particular site or culture.
Ideally you should try and find a particular topic that interests you, and make sure you have a fairly succinct title to focus on, so that you can keep to the point fairly easily, but have plenty to say about it.