So now I feel comfortable with OOP, I have tried a little bit of QT and wxWidgets, but I ended up doing the same projects that I used to make but now with GUI, which resulted in me feeling like I'm not improving at all.
I don't have anything specific in mind that I want to do, is there something more general that I should learn to improve my C++ knowledge?
Although, I would also look into more advanced features of the standard library. It is massive, so look into algorithms, threads, data structures, etc. You can do a lot. Template metaprogramming and SFINAE is also very nifty and powerful feature of C++, so that could also be something you look into if you want to design powerful template code.
I agree that it is so important to learn standard libraries and its extensions (STL & Boost).
But, OOP is too fundamental & essential to look lightly.
If you have not yet touch,
effective C++, and so on, critical books should be read, and accustomed to designing your own class library.