I find that my strongest inspiration tends to come from the smallest places. My last three novel ideas have all started from a single, strong image with a feeling attached. Where these strong images come from has varied. One came from reading two very different books back to back and having them clash in my mind. Another came from a physical place I was in, imagining it 100 years in the future. The last one, I don’t even know where the image came from, it just kind of appeared.
I find that as long as there’s a strong emotion attached, that’s fuel that I can usually spin out into plot and character details.
Side note – for short stories, I have this same problem. It seems like everything I come up with wants to be a novel – which is such a shame because I really want to write short stories too!