Make something on a unique topic where you can be the best in your field and really fulfill a great need. Most forums die within a year of starting. They are easy to make but hard to make active (in my experience). I am about to start a new one.
I might also say that wanting to start a "forum" without having a topic in mind is not a good way of going about things. Like tajhay said, I have found my ideas do much better when I need to use my own forum often and I am really passionate about the subject it's about.