In what way do the images need to be improved/enhanced?
If this is about increasing detail rendering, there's not a lot you can do. There currently are tools to up-res images using AI technology, but these also cannot make up detail that wasn't captured in the first place. Neither Darktable nor GIMP offer such a feature.
If these were old film images (e.g. slides) when you scanned them, I'd consider just leaving them as they are. You could optionally adjust the colors a bit to come closer to what the original scene might have looked like.