If you're looking for a service to give you a map of topics and subtopics for your niche, Nichely is worth a look. It's got AI tools to research a subject, including maps of topics and clusters of related long-tail keywords. That can be useful for content creators, publishers of niche sites and entrepreneurs trying to establish topical authority and optimize their content for search engines.
Another tool worth considering is Heuristica. It's more of an AI-powered concept mapping tool, but it uses a mind map interface to help you organize and visualize complex ideas. That can be helpful for researchers and students trying to better understand and connect different ideas within a subject.