I hate having a seo/webmaster related domains. It's impossible to find a good term that does not have some other seo dude who charges 100000 a hours using his 10 other pagerank9 sites to rank high in all seo related search terms.. ggr
You can carry out some seo techniques to promote your website to pr10, and then you charge others 100000 for placing links on your pages too. They can do it, and definitely you can also do it.
Having a webmaster / SEO related site is not exactly a niche category. Maybe you should try developing a site for an actual niche, where there is not very much competition.
I feel you! I have this german website promotion blog with some authority and mainly get long tail traffic. until I accidentally used totally off-topic keywords that tripled my traffic overnight, that is... well, at least adsense on that post pays my hosting