Great Question. My last published hub was June. I am seeking 'what' to write about today too. My focus for my search has been trying to discover through exploration a 'Niche Topic' for a Hub Group of hubs. Traditionally the hubs I produced were not real traffic builders beyond the Hubpage community market or with social media. I really do not try to promote them beyond the initial posting.
However, that does not answer your question :-) To be honest I write everyday sometimes for hours naturally in my PC. Then I seek editing them for quality and length. 'What' I write about generally is what jumps out at me or a continuation of something that did or a different take on it.
What jumps out at me comes from a practice of making a list. I wrote about it at one of my hubs. What I do is I choose any topic even if it is 'Why am I happy', 'I love the rain' or 'Drag Racing is cool'. Then the first entry for the list is Supercalifragilisticexpialidocious. Now, I enter as quickly as I can something about the topic in a word or more no longer than Supercalifragilisticexpialidocious. Keep it short.
Next, I walk away. Maybe poke my head outside and get a fresh breathe of air or some such as long as I wish. Eventually I go back and read my list. Sometimes the list itself is an article structure or parts of. Maybe one entry will spark a flame too. Then I (research) write with that as inspiration.