Niche subject in your forum site is very necessary for getting the quality back links in very less effort but if you don't post any niche subject in the forum accepting post on general subject then you will have to more focus on the multiple keywords to rank well in the search engine.
IMO I would choose the topics that receive the most traffic and posts as your niche and focus more on that since your users seem to like it better, then base your SEO around that. You can still have the other off topic forums!
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I believe your forum do need to categories every thread should go to so you able to go for the niche. Or you just pick what famous topic of most discuss topic keywords as your target keywords and get dofollow blog niche to do backlinks.