Think of something that sucks. If you feel that way, others likely do as well. If you can find a way to fix it, some percentage of those like you will pay for it.
The levers to pull on are 1) how many people are impacted by the problem, 2) how expensive it is to solve the problem and 3) people's propensity to pay for a solution.
Hailing a cab in SF on a major holiday sucks; enter Uber/Lyft.
Staying in hotels with your family sucked, people wanted to stay in a home; enter AirBnB.
As a sales rep, tracking deals always sucked; enter Salesforce.
Going to the grocery store sucks(a lot of times); enter Instacart
There are no shortage of things that suck, just people willing/able to solve a given problem.