At the moment, Export Group Layer into images doesn't trim images into visible area, and there are cases where you wouldn't want that to happen. Thus, I think an additional feature for this would be a way to trim to visible. It would enable one to have something akin to Image Slices via my tutorial on reddit without the need to save and keeping it as non-destructive as possible - https://old.reddit.com/r/krita/comments/8wd4ic/krita_tutorial_image_slices/ .
I forgot to mention that the trimming should either be canvas-based (fully non-destructive) or deleting pixels, but trimmed to visible (not fully non-destructive). I guess that makes 3 dropdown options. 1. No trims 2. Canvas based trimmed using visible data on group layer 3. Pixels-removed trimming using visible.
And finally, I thought that .kra and .psd should preserve information as much as possible. So, that should be another feature in mind.