One thing I have learned from past experience of owning my own forums is that so many genre's of forum are now overly done, meaning that depending on the kind of niche you choose for your forum you could have high competition. When choosing an idea I always find it best to search around for other forums of a similar niche, take a look at what they have to offer and think of what you could offer that is more unique. If you can think of unique ideas for a forum of that niche then go for it and put your idea into action. Without unique ideas your forum could end up getting lost in the crowd which is something you would want to avoid especially with a new forum.
How do you make a decision on an idea for a forum? Is there any kind of protocol you follow before jumping in with your idea?