If you want my two cents, I'd say look at what your genre is for your forum, and then look at what your members respond to. Even if you have 10 members, let's say, for argument sake, look and see what the most active ones are doing. Then get an addon to cater to them. If they like discussing items, parts, pictures, etc, a gallery would obviously be a good idea. Help them get familiar with it, and make sure they KNOW it's available. Then lead by example.
Continue in that way until you figure out what they like, make sure they're integrated with it, whether it be quickly or slowly, such as giving them permission only to view galleries in the above example, and then go from there. Eventually you could give them the rights to make their own gallery. That'll obviously drive interest in your forum and people will push for quality because they enjoy whatever addon you're giving them.
News type addons are always a nice basic feature (such as the Library by Waindigo) or the Social Groups addon. Those both can be a nice starting point that COULD potentially raise the quality of your member's posts and content.
Not saying it will, it just might.
HTH
