Write Down Three Passions
They could be ones you are very knowledgeable in, or ones you wish to learn more in, as long as you are truly passionate about them. Maybe you love eating, you could do a niche on food. Perhaps you love animals, or soccer.
Don't worry about not knowing all the answers of your niche, if it is something you love, then learning about it will be FUN, and the information will be easy for you to attain. I have a site about rock n'roll. Do you think I know ALL the bands, albums, stories, etc. about this HUGE genre? Absolutely not, not even close. But I have a huge love for the music, listen to it constantly, and I have been writing content, and learning so much on a daily basis.
Maybe you love food and want to learn about healthier options for your favorite dishes. This would be beneficial to you, and to so many people on the internet. You could create your own recipes, and then maybe even market and sell them!