One way of brainstorming my English teacher this year had us do was to come up with a list of things that you have something to say about, pick whatever resonates the most, and just freewrite until you've said everything you wanted to. Look through it afterwards, and there should be an idea or two worth developing further. Revising might take a while, but it's definitely worth doing. I went from roasting Brutalist architecture to going on an edgy Existentialist rant to describing why I love reading to talking about the fact that I feel confined by Chinese-American culture and how I discovered my talent for sculpting, which is what I ended up going with.