Making a forum that was supposed to be a revival or successor of another one. I have found doing this is not always a good idea or concept.
I had some people say they would sign up, and never did. If they did sign up, they never posted or were active. I then closed that forum down and decided I'd make Discussion Block which is a general discussion forum. So far the same thing has happened on DB. People have signed up, but barely anyone is even active. 50 members, and like maybe 2-3 are active if even that.
I've decided to never ever do successors or revivals of any forums that had been once active, but then closed down. To be quite fair everyone who was a part of GameTalk, GTX0, and similar have moved on with their lives and onto other things.
I once had a crypto niche forum. When I started, a lot of people said it was a profitable niche. However, when I started I found that it was a difficuylt niche to get regular activities.
Doing contests and games. On Christianity Haven, my members weren't interested in either because they want good discussion. I spent a lot of time (wasted actually) coming up with some fun things to do, and then I'd have 2 members participate. Not worth it in the end.
This is a different take as I've seen people encourage others to "start their own forum". So my answer is: Starting forums in general. I still believe in forums but some people create them with unreal expectations that all they have to do is "Create & they will come". Back in the day, forums were more active & it was easy to get members. Nowadays, admins need to work extra hard to get & KEEP activity going.
If one wants to start a forum, they have to realize that those successful ones out there are admins who work tirelessly.
I wanted a site like Webmaster Babble so I created Fun Biz Forum, but I never took it to the max, but a few other forums promoted on here by other people have done well.