There are so many things to write about. Exploring this site - especially the topics section - and hubs written about the topic will help. I have written a hub called "Where to Get Ideas for Hubs" that gives you some ways to come up with ideas.
I would recommend thinking about what kinds of things you know very well - a skill, a hobby, something from work that is translatable to a mass audience, that you can teach others. Work on a specific "classes" around the topic.
Like CJ said, make a list, and every time you come up with an idea, put it on the list. For me, the issue was not that I couldn't come up with ideas, it was that I would critique them too quickly as being "not hubworthy."
When I was in the challenge like this one, I decided to simply try out the hub. I would find my niche so much better when I wrote whatever I was thinking about instead of forcing myself to think about something else.