That's a good question, but your kind of looking at it backwards, which is what a lot of people do.
Trends are a bit different. Trends are trying to predict something and that is not the easiest way to start a business. It is a lot easier (and cheaper) if you are a little behind the curve, picking up the crumbs of the big boys and in evergreen niches.
Then getting lots of crumbs means you end up with your own big pie without the risk.
The single most important thing is how are you going to drive targeted traffic to your store, because without targeted traffic you don't have a business.
So the first thing you have to figure out is where are you going to get the traffic from.
The point of drilling down to a sub niche is that makes it obvious which traffic to target.
Once you have done that, just use keyword tools as if you were doing seo to determine demand and look at how many relevant social media profiles you can buy shoutouts from. When you do that demand levels become obvious. But core niches never die you just need to drill down to a sub niche and give it your own spin.