Steve Blank describes a Customer Discovery methodology that starts with a Product Hypothesis and then tests and refines it. But what if you aren't at step #1 yet? What are some good ways to generate hypotheses?
Anyway, my input is, based on what you know best and your experiences, come up with something that makes sense. Keep developing the hypotheses until you cannot poke holes in them or have honest doubts about them, then go test your hypotheses against anyone who knows what they're talking about other than yourself. If your hypotheses are still intact, then expend the cost to reach out to a potential customer. Saves you a lot of hard work.
Take the intersection of the three things you know most about and are passionate about. If you're not sure about this simply look at your most visited sites.
Then use those are your "base" and try to find ideas that combine all three.
Anyway, my input is, based on what you know best and your experiences, come up with something that makes sense. Keep developing the hypotheses until you cannot poke holes in them or have honest doubts about them, then go test your hypotheses against anyone who knows what they're talking about other than yourself. If your hypotheses are still intact, then expend the cost to reach out to a potential customer. Saves you a lot of hard work.