Nice suggestion, but I don't think there is a need to duplicate image editing software when there are already so many editors available. Simply edit the cover image in your image editing software, then import back into Calibre.
Plus adding a graphics editor to Calibre would be a major task, for such little benefit. Not all covers can be fixed with an eraser and paint brush. Some covers need some pretty substantial work.
See one of my examples here...
https://www.mobileread.com/forums/sh...d.php?t=342846
It would be near impossible to do this with basic software. (fyi... that example was recreated, not edited. The only original component is the silhouette of the man)