Hi,
If you want to focus on multiple niches using one domain, you have a huge task ahead (while this is highly recommended rather than coming up with multiple niche websites focused towards their corresponding niche). You would need content of great quality to cover all those niches. If you wish you make this domain to be able to rank high for different terms in multiple niches, you should prove the credibility of your website in terms of content quality and usefulness of it to the visitors. Take an example of websites like Wikipedia. They rank high for hundreds if not thousands of niches. So quality of the content is the key here.
Websites that try to target multiple niches with thin content will go no where in search engines and will be flattened within no time. By the way, big and authority websites with quality content can out perform niche websites (focused on a single niche) in many ways.
Hope you got the idea my friend.
Best,
Devanur Rafi