For Wikimedia Commons the issue of how to handle digital restorations has been discussed several times, there are no single solutions. The general position is that original images, such as very old glass plate prints from archives, should be preserved and displayed in their original state, but there is no harm in making available restorations as separate files that fairly and accurately represent the photograph and remove damage or flaws. The reuser or reader should never be in doubt that they are looking at a restoration rather than the original, and we have to keep in mind that we do not always know whether the reader's monitor, display device or printer may also adapt the saturation and brightness of the image.
Though it is possible to do large batch changes for light levels, sharpening, colour correction and so on, these remain controversial if the original file is overwritten. This is especially true when an artwork has been photographed or scanned.
Among recent on-wiki controversy is colorization of photographs, especially old black and white photographs. These may be interesting, and at times useful for illustration, but again the reader should be in no doubt that they are not looking at "reality" and as good practice it should always be easy for the reader to navigate to the unenhanced original.
Fae