I'll write a hub on almost anything. If I don't know anything about it, I research it until I'm an expert in the field. You can get ideas from anything, my personal inspiration comes from watching tv.
For instance, I have a few hubs about facebook games. I became an expert in those games by playing a ton, then wrote in depth about them.
Another way to think up hub ideas is to say to yourself, "What do I shop for?" Then you can create a commercial hub with Amazon products.
Hope that helps!