If you're looking into building toolkits for not-too-popular languages, I feel like Erlang should be interesting, the process model sounds like a perfect match to GUI components. I've never seen anyone trying to build that, for some reason. I have a feeling GUIs with borrow checkers are hard to get right, wouldn't advise building Rust toolkits as practice.

Rust is expected to be more-commonly used, cause it better handles the large domain of C/C++ based code, as it is less error-prone, but still fast and energy-efficient, which. Linux kernel is starting to support Rust now.

Rust for instance has no overhead of garbage collection, while Erlang does garbage collection at the end of every function.

It would also help you learn Rust, which will likely be a more in-demand skill than C++ in the future.

I've got a little Rust GUI library that's pretty young, and relatively simple, so maybe fun to contribute to: https://github.com/audulus/rui :)

So much this, a UI that understands traits, structs and automatically displays a control for that type in both bulk edit and single form mode would get money from me.