I would not to create any micro niche site for adsense now, it's just not worth for my time in my opinion, but if you plan big and dream big, you gotta have enough resources to make it happen. I wish to have another new niche giant authority site, but the problem for long term is content creation. Assume that you are good enough in SEO, the problem is content, will you study the niche yourself and create content? Initial content is easy, outsource it, but for long term, will you still hiring them? some niche require heavy initial investment.

If you're planning to build an authority site, I definitely recommend getting to know your niche, target audience and the market in general.
Not that it is something you can't build a site without, but knowing what you're talking about is generally a good thing.

For example, if you know nothing about fitness and muscle building, how would you know that the content you just received from your freelance writer is of good quality? I mean, okay, it is grammatically correct but you have no way to tell if that would be useful for your reader or not.