This is how I've done things for years now.
For me, the old "find something your passionate about" doesn't cut it and selecting a niche, then finding a way to monetize it feels like you're working backwards.
When I go through affiliate marketplaces, I always start with selecting a product I want to promote FIRST then going back to conduct some niche/keyword research, and putting a marketing plan together.
Doing it this way I've gotten involved in some very profitable long-term promotions that I would have dismissed completely if I was just looking at a few specific niches or thought I needed some level of "passion" to get involved.
Most of this stems from when I first got involved in Internet marketing and was promoting a physical product for the inventor (he was a local guy). The niche chose me. I just had the product and some bullet points to go on. It was up to me to build a site, find the audience, and drive sales.
I take this same mentality when promoting digital products as an affiliate. I let the niche choose me by selecting a product that looks viable. Then I think along the lines of: "this product creator has commissioned me to sell their product, how can I reach a targeted audience and sell it effectively?". (Basically, you can treat this like you're the marketing company they chose to promote their product.)
Some of you may be doing some version of this already, but I don't hear it talked about too much outside of building 'sniper' style sites or launch jacking that's why I wanted to throw it out there.