Don't be afraid to be a little controversial with your content.
An emotional trigger will encourage people to discuss a topic. You might need to adjust your forum rules, though. Maybe loosen them up a bit if you plan to tackle emotional triggers that might anger people a little bit.
Other than that, go visit other forums, not just ones like your niche. Visit any active forum online and see what posts on their platforms are getting the most engagement.
Try to figure out how you can use their most engaging content on your own with your niche.