It sounds a bit plain and boring, but if you're just wanting to learn the basics of C# then you could refer to the official Microsoft docs:
They're quite basic, but do offer some fundamental knowledge. If you're after more theory such as architectural patterns, I'd say you're better off looking at resources that are language-independent
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.