I've started a small experiment to see how difficult it would be to port our file chooser dialogues to "native" versions (GtkFileChooserNative). This would mean you'd get your OS specific one when opening/saving a file, rather than the current GIMP default.
So far, I can see that most of our dialogues can be trivally ported (like the QBist plug-in, screenshot from Bruno Lopes).
Unfortunately, the main image export dialogue will require a full UI redesign. More to do after GIMP 3.2!