Starting a forum isn't easy. It's not just a matter of coming up with an idea -- you also have to come up with a way to get visitors (and more importantly, visitors who participate). There is nothing more pathetic than a forum filled with nothing but the forum owner posting desperately trying to get somebody else to join the conversation.
And if you do manage to get the forum off the ground, you will need a whole team of people to moderate, and potentially a couple of administrators, too. Trust me, I'm involved in several active forums, and this is not something you can handle on your own. It's a lot
of work to run a forum.
Frankly, you might do better to start a blog. It's easier to get started, and you don't have to worry about attracting large numbers of active forum posters right from the get-go. Find other bloggers in your niche and start networking with them. The built-in RSS feeds and ping/trackback functionality will help get the word out about your blog and build your readership.
If your blog becomes successful, you can open a forum then (or maybe decide the blog is so much fun you don't need the hassles of a forum after all).
No matter whether you start a blog, a forum or a traditional website, it's going to take work to get it off the ground. Hard work, and lots of it. You'll find it a lot easier to put in the time and energy if the site is about something you find interesting in the first place.
So pick something you're interested in. Something you feel passionate about. There are people out there who are interested in almost anything you can imagine, so you don't have to worry about not having an audience once they find out you're there.