It doesn't need to be profound. Could be a question you Googled at 2 am, something a stranger said that stuck with you, or a topic you keep meaning to explore but haven't yet.
In my case, last week I found myself thinking about walking (a lot). It's my favorite hobby, but at some point, I realized I'd never actually looked into the science and psychology behind it. So I started wandering the internet for resources, and before I knew it, a whole newsletter edition had written itself.