That's basically an article directory. I understand you're planning on writing all the content yourself but same principle still. Rank for lots of different things, across multiple niches and make money from ads. Article directorys also generally rely heavily on the content and large number of pages on their sites for rankings.
Yes, you can rank for lots of keywords if you stick to the right keywords and build it into an authority site. Internal linking can be very powerful if done the right way. Also you can certainly make money with Adsense ads but it's about the lowest paying way to go. The upside is that it's also the easiest way to monetize a website.
You might think about testing with affiliate or CPA offers too even if you haven't had much luck with affiliate marketing. If you present it more as a helpful resource rather than a sales page you're more likely to get affiliate sales. For example: "How to Get Top Rankings - I do this, that and that and I used this tool, plus this other tool and this service. Here are the results I got..." Then each link to a tool or service is an affiliate link.
I'd say it's easier if you work within one niche though. Especially if it's something you already know a good bit about or are interested in. That just makes the writing easier to get done. If you pick the right niche you'll never run out of things to write about.
Just go a little broader if you don't see enough easy to rank keywords is all. I've worked with a large number of very small niches and still found thousands of good keywords to work with.
