My advice would be to narrow your focus a lot more. If you try to have a subforum for politics, another for scuba diving, and another for Corvette C7's, you will always lose out to forums who specialize in those. Not to mention it would be impossible to try and market the site.
You are just joking. This is not possible where you can discuss everything. You must have to specify topic.
There are so many general discussion forums already you may find it difficult to get one to take off. Your best bet would be to find a topic that interests you. That way you can build the forum with relevant content that people would want to read.
trimming down your forum to a specific niche/ topic would be a good idea.... you can search specific keyword that are often searched in search engines to identify a topic, try to choose topic which are in demand. however if you would insist on having a forum for general discussion how about straightofftopic.com, i'm quite sure the domain is available (just checked godaddy) however i'm not sure if you would the price.