[jmduke]

I am curious as to your phrasing of "besides by doing market research" — market research is the whole point!

Most successful API products in the sense that you're describing share two common characteristics:

1. They are a thin wrapper around some dynamic dataset.

2. The cost to an organization maintaining that dataset is prohibitively large and/or difficult relative to the value they receive from it.

If I were in your shoes, I'd find a niche that I was relatively well-positioned to address (say, Shopify stores, or programmatic SEO — something small enough to have a uniform opinion but large enough such that there are multiple cohorts of individuals willing to pay) and spend time in their forums looking for people complaining about datasets or information they wish they had. The API part is an implementation detail.