Are there companies working in that niche? Are there people buying things related to that niche? If so, you can generate money from several ways.
1) The traditional AdSense ads.
2) Have companies buying ad space in your forum;
3) Have merchant/sponsor accounts for companies to promote their products (you charge a monthly fee). Don't allow promotion on signatures, or selling.
4) You can set up an affiliate system in-forum. Imagine that your forum is about flowers and people are talking about fertilizers, so whenever they talk about a fertilizer that exists on amazon, it will automatically create a link to the product, if people buy, you will make them moneys.
5) Merchandising, white branding/forum brand products. If the products around your niche aren't very VERY expensive and hard to source, you could create a brand with products that people usually want or need, or a product that it's too overpriced. If it is something like space exploration, you could stick to the old way and do the common mug, shirt, stickers (this is more helpful than anyone thinks) pen (or others, but you get the idea).
6) Grow your socials and have sponsored posts there.
7) If you feel greedy enough and you have a lot of golden content, create a paid part on your forum, I don't like it, I don't advise it, but in some forums, it can work. Ferrarichat pulls it off, but people there aren't counting pennies, aren't they?