Hey! Guess I'm a little late to help you since school's on the way (or has started).
I personally have a very clear idea of my theme and even my final pieces. In fact, I chose Studio Arts as a subject after subject selections at my school because I really wanted to make a certain piece. But I know a lot of people who have no idea, so you're not alone. I'd rather not specify my own theme, out of paranoia that someone could do something similar by seeing this thread and ruin my chances at Top Arts.
I've heard it's good to combine several things together. So maybe you could combine your interests that you listed into some cohesive folio. In terms of fashion, do you also do textiles or embroidery? You could just choose fashion and figure out what you want to focus in later. As someone who's done 3/4s, there's time. You have to work hard, but not everything always works out immediately.
Good luck to you!