Work in some unrelated industry and find out enough to know their problems then make a web app that helps them. With businesses or professionals as customers, you can charge much more and with such a uncrowded niche, you can do a relatively poor job.

An example, I've seen is a site for property managers to share information with their landlord customers. It's just a CRUD app with a list of transactions, area to post pdfs of invoices, etc. The web app is old, ugly and basic but I bet they're collecting big regular subscription payments from their customers who can't justify hiring their own web developer and building and maintaining their own site.

The key here is having knowledge that most web developers don't have. It's going to be an uphill battle trying to sell golf clubs or restaurant bookings or solve any problems that everybody and their dog understands.