I look for forums on topics that are similar to known profitable niches, but different. The parenting niche is a known profitable niche, but there is no online toy forum. You need to find unexploited topics.
Another idea is to find what your niche is missing or fix a problem in your niche. Bring something new into the space. For example, GothamChess and Fireship bring entertainment to the chess and programming spaces, spaces that are historically boring. They took an existing niche and improved on it. Another example: Amazon improving on the self-publishing space with POD, making it more accessible. Find a problem and fix it, and people will come to you.