hard to say. I would do something that doesn't have a lot of competition and not a lot of people looking for it at the moment. What you want is something that will eventually have a lot of people looking for it. For example, an upcoming movie, tv show, video game, new car so on so forth. That way you can quickly move up to the top of your desired serp (ie. [your keyword] forum) with minimal effort. Then when your keyword becomes popular you will have already made it to the number 1 spot and you can start reaping the benefits of being early to the game. Also you can get early dibs for the good domains like [keyword]forum.com. You want to rank early in "[your keyword] forum" on the google serp because those people are the ones who are looking to join a forum for that particular topic. You might rank well for a lot other odd ball keywords but those people aren't going to join, they just want the information that is located on your forum and then leave. The one good thing about forums is that they don't need much maintenance when they are small. Just set it up, seo it, and forget about it. Then when people start using it you can start moderating it. One thing I learned from running forums is that you need to be early to the game. Just imagine trying to set up a webmaster/seo forum when you have to compete with digital point forums? Your gonna have a hell of a time doing that.
All subjects are just as easy, as it all depends on the forum software to set up, rather then topic. Though, by reading the end of your post, I assume you meant topic, so I'll give an equally pointless response. As an admin should be active on their forum, and understand the subject matter, it's VERY important that they start one on something they enjoy themselves. You also need the time and be be willing to put in the required effort to maintain it, promote it, and to be active yourself.
That is the weirdest piece of advice I have ever heard. Anyways, you need to start a forum about something that's in a niche without a whole lot of competition, but more importantly, an area that you care about and like. That way you'll be into it more.
Webmaster niche is crowded imho Find something not crowded, but still popular... Online game forums are popular as hell
Like mentioned before, start a forum in a niche you like and know a little bit about. This would make it easy for you to help your members on the site, provide information/advice....etc It's hard to start a forum and keep it interesting for your members to come back. SEO'ing won't make it a success (Although helps in search engine traffic in long term). Also, if you are going to start a forum, i think it's best to get a custom made design and not use pre-made templates like most people. Stand out from your competitors cause the forum market is a hard place to jump into. Some forums die after a year cause the owner neglect's to maintain or help keep it active. (i'm having a custom one made and it's almost done along with a few more changes)