One class of markets that I suspect is a target-rich environment: disability-adaptation tech.
1. There is a decent chance that existing entrants are tied down by a focus on meeting the regulations rather than on delighting users.
2. You might be able to significantly scale-up and thus improve the unit economics.
While the market of people who can pass the means-testing to qualify for whatever government-subsidized benefit exists might be small, the nature of bureaucracy means there is often a MUCH wider pool of people who are "quasi-disabled", either permanently or temporarily.
Note that there are a few disabilities which are so overwhelmingly common that the market is already saturated. The classic example is eyeglasses.
3. The specific disabled population is a ready-made population of early adopters IF you identify that you can keep your focus on delighting users and just use the regulations to remind yourself of risks and edge-cases. (This really will depend quite heavily on the quality of the regulations -- notice the difference between Japanese zoning and SF-bay housing-approvals) Lots of legacy disability tech:
* Has a pretty frustrating maintenance cycle, so you can win on a strong customer service brand IF you can innovate on operations.
* Can be pretty frustrating to use, so you can win big on design.
A focus on delighting users rather than ticking boxes is naturally going to produce a naturally better product. Here is the key tactic though: lean on the early-adopters for high-detail feedback. This problem occupies a much larger proportion of their lives than the average user of a product.
This higher-strength signal of user needs is known as the "Curb Cut Effect". https://thingofthings.wordpress.com/2014/11/15/the-curb-cut-...
This also seems to hint at a good trick to turn any regulatory box-ticking exercise into purpose-driven-design. After all, a focus on privacy-by-design is better than a focus on the letter of GDPR-compliance, right?
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Hilariously, I used to work for a YC company unwittingly building assistive tech for my particular disability. Their Curb Cut effect was so ridiculously strong, they've occasionally written ad copy with reference to a symptom I've experienced and I'm pretty sure they still don't even realize it. Looking back, I now really wish I had been self-confident enough to explicitly disclose -- especially when their CEO started using a wheelchair.