Most people asking and looking for hot product to sell online, i think the good idea is who exactly you want to serve, the more specific in their interest,like and needs the better.

Look for community or group of people with the same interest and need, know thier problem or pain then provide them a solution addressing thier pain using your product or services.
Sample: Community of people around fitness.
Do They want to improve thier health, lose wieght, gain more energy, fix thier posture, improve thier body shape..etc.
The more specific on thier wants and needs, the easy to find a product to sell.