Internet marketing ain't a sub-niche; it's one of the biggest and most saturated major niches on the web. Anyway, I have a friend here on DP who's running a startup business forum and he's looking to make it the biggest or one of the biggest ones on the web. You'd do well to stick with a forum idea that you love, as Nathan suggested. Don't try to force a forum on something you don't know crap about, as it doesn't work as well as blogs and articles wherein you can have people write for you. Forums work in a slightly different manner, even though you can still hire ghost writers for your forums...