I write a lot about the United States,its problems, my hopes for it. The news is always triggering my brain into something. I also write about household stuff, mostly kitchen related. Sometimes I delve into history. Do you remember having to write history reports in school? The history reports you wrote in school we perceived by a person with little life experience. Look at those same topics again through the eyes of the person you are now. This to make comparisons. In the end you may not write about history but you may write about a new perspective the you found while looking back at history.
Other than that, write whatever. Moments of silliness, things that make you laugh, things that make you cry, or anything else. When you sit down to write, write whatever. If it turns into a hub, great. If it doesn't, don't worry it may grow up to be a hub one day. Even a bad idea or what seems like a bad idea can become a great work if it is shown the light of day.