If a user exports an image to a file format that does not support all the data contained in the current image -- channel depth, colorspace or layers etc -- then the export dialog should warn about the degredation.
At the moment wantExportConfirmation is overidden in KisDoc2. Changing it would present a dialog for all non-native save or export actions. This isn't very fine grained, but there is no format that has all features that Krita supports (except for OpenRaster maybe)
Well yes and no. If you have a single layer RGB8 image. There is no reason why you should warn the user when saving to PNG or TIFF. Besides the dialog should also suggest action. For instance when saving a CMYK image to PNG, it should offer the list of supported color spaces in PNG, and it's probably a good idea to have this inside the current option dialog instead of a different dialog.
Maybe this should rather be made a wish instead.