The games-emulation/dolphin package currently does not have the qt5 USE flag as a requirement. When compiling it on a system with USE="-qt5", you can't run dolphin-emu. Trying to run it roughly looks like this: user@localhost ~ $ dolphin-emu Warning: Ignoring XDG_SESSION_TYPE=wayland on Gnome. Use QT_QPA_PLATFORM=wayland to run on Wayland anyway. qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in "" This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem. Available platform plugins are: eglfs, minimal, minimalegl, offscreen, wayland-egl, wayland. Aborted When setting USE="qt5", it works. As the dolphin emulator currently doesn't have qt5 as a USE flag, my proposal is to add it for functionality purposes.
It has a USE=gui. What happens when you turn that on?