When you sold the first company, how did you pick the subject of the second company? Was it an opportunity, or was it something you really loved? Is there something you are really passionate about now? Could be anything… hobby, travel, certain pieces of tech, whatever. Think about it and try lots of things out. Maybe go back to things you enjoyed when you were a kid. In my experience, you need something, or a series of somethings, that really interest you and drive you. They might seem silly. They might seem dumb or done already. See if you can find something that you like to play with. Really play. Go from there.