One problem that has long plagued X11 is that any app can snoop on any other app, including things like keystrokes and displayed information, even from within containers like Flatpak. (This is understandable, since it was designed at a time when spyware was rare, so there was no need for isolation more fine-grained than the user level.)
IIRC, Wayland didn’t address that problem in its early days, but in these modern times of surveillance capitalism, I suspect it has been getting more attention. It would be nice to see it solved.