That's the path I took with Polygonjs ( https://polygonjs.com ), and a game I've just released ( https://polyreplay.com/minesweepertwist ), with more coming shortly.

But it didn't start like that. It only started as a tool I could use to deliver client projects, as I was trying to become a freelance for interactive 3D scenes for the web.

Project after project ( some examples here: https://polygon-lab.com/ ), I could improve Polygonjs. Then I found clients who would be interested enough to buy licenses, and would give valuable feedback which would help the project grow even more.

And a few clients asking for not just interactive sites, but also games. This pushed Polygonjs further, and after several games released, it definitely qualifies as a game engine.

So this is generally an advice I give to people who want to become freelancers. Build a tool that solves a problem in your space, as this gives you an edge, and you'll also get the chance to confront that tool to reality, which will help it - and you - grow. This becomes a virtuous circle very quickly.