I know there's Pygame, Java Swing, and others, but is there C++ GUI being planned for repl.it? I would love to use something like wxWidgets.
Natively, no. It can be done, but it's a bit more complicated as you'd have to use a polygott repl which uses makefile syntax and compile from the linux source code.
EDIT: I didn't see that you said planned. I don't know if one is planned, it'd certainly be nice, you can ask for it at https://repl.it/feedback.