Well I can't think of something that I am passionate about that would actually work out. I mean I have tried a fish forum, and no one liked it. It failed within the first 3 months. I might try a yugioh forum, but i don't know how that would do.
Whatever you do, never even think about using a niche that you aren't interested in. If you play The Sims, create a forum about The Sims. If you have recently become interested in a new graphics software, create a forum about that graphics software. You want to find a niche that you are passionate about and that others like you are passionate about. List some of your interests in this thread, and we'll give you some feedback on them.
If you are a true and dedicated Yu-Gi-Oh fan, you would be able to name all the characters off the top of your memory. No good starting a forum and a member asks about this monster and you have no clue about it, you know?.