So usually what happens is that an individual is an expert in their under-served area, sees the opportunity, and then puts in the effort to build a business to serve that opportunity. The point being that you won't get people who know of lots of opportunities, but rather many individuals who each know of their own opportunity. And you most likely won't find such domain experts on HN.

It might be worth looking for evidence of attempts at building such businesses, since often such domain expertise doesn't pair with business-building expertise. So if you're trying to build such a map, then maybe look at all the businesses (including all the failed ones) that served each market.