How do you come up with writing ideas for Hubs?
Requests are a good start
As the subtitle says above, requests are a great way to kickstart ideas. Just read through the main Q&A page and look for questions about things you know about.
If you're looking to write non-fiction, a good way to start is to think about things you are interested in. Consider questions you might ask about it, or would have asked about it if you didn't already know the answer. Do a little research, and as you acquire more information on a subject, the ideas will start flowing.
Reading the daily news is also a good idea, if you like to write opinion pieces. People probably don't need you reporting on what's already been reporting, but there is demand for intelligent opinions on current events. Take a look over the first page of a news site or paper and see if there's any stories you feel strongly about. If it's something important to you, the words will come easily.
If you're looking to write non-fiction, there are a lot of writing exercises you can do to break up a writer's block. There are many of them around the Internet, but some of my favorites can be found on WEbook:
Their 911 Writer's Block page has numerous ideas to get you started.