There are plenty of problems that haven't been solved. It's just that most of these are million dollar ideas and not billion dollar ones - VCs won't cover them. They're too risky to do as a team, but too big to do as a side project for one person.

There's space for converting reference books to reference tools for example, like a city council once asked me for something to make building valuations easier, but we never closed that deal. Android is missing a dumb cloud note taking tool that just works, and yet there are a trillion note apps for iOS. Heck both platforms are lacking things that just charge once, no subscription, no gacha model.

Also AI opens up a lot of previously uncovered fields, and a lot of it is just API connections.