I've had a very similar issue recently - when I joined my current company, it was a start-up and had a lot of room for growth and experimentation. Now it's maturing and I'm looking for personal projects in a similar way.

Funnily enough, I've actually been using ChatGPT to generate specifications for me which are usually fairly good. If I'm looking to play with or learn a particular technology it also does an okay-ish job of suggesting a 'product' with a specification that works well for the given technology too.

Outside of ChatGPT though, I occasionally just browse through GitHub for open source apps that I can get a rough idea from and attempt to reproduce as well. There's also lists like this one which suggest different project/app ideas at varying levels of difficulty: https://github.com/florinpop17/app-ideas

I can't say I've found anything on GitHub that's a proper list of specifications though.