It would be cool if GIMP's new format could remember the undo history. I sometimes save edited images as XCF projects so I can quickly make a tweak and re-export it if I need to, but if you do something like crop, color grade, etc., and then save it, you can't undo that later when you reopen the project file because the original, unmodified image data has been lost. You can work around this somewhat by saving unmodified versions of something to its own disabled/invisible layer, but it would make things easier if there was some kind of undo history preservation so you could open a project and sift thru the history of operations performed on the image.