Well, the only forum I can think of that works, worked because people needed it. If you want your forum to work you must make sure people find what they want and it's not an exact copy of something else. You need to have a specific subject and make sure you can provide the users something they can't get elsewhere. If you can do that, I assure you the forum will work with plenty of users.
If you really think hard, you can think of a good subject . But always choose something you know lot about . Oh and content is the real part. The more content the more activity!
Haha it's funny you mentioned that, I'm getting a music site/forum getting started up as well. Mostly centering around Hip Hop/ R&B. It's good you realized that there are WAY too many GFX sites, gaming sites, etc. The whole point about a forum is to see a demand for information on a particular subject, realizing that there is little to no competition, and than creating the site/forum. One of the reasons why Gamerenders was so successful was because it was one of the first forums to be created about renders/GFX.