I would like to see a feature for the Media Library.
Some image hosting websites I know have the ability to replace an already uploaded file by just uploading a new one on the file's page inside the user panel. The URL is kept the same so you don't have to worry about updating the pages where this image is used.
I'd like to see this in WordPress as well. That you can open an image in the media library and choose [UPLOAD A NEW IMAGE]-button to simply replace that image with an improved version of that image.
The way it's currently done is a bit devious. Upload a new image, remove the old one, make sure the url's on all pages that uses the image are updated. It's easily a job that can be improved with this feature.
And of course this feature should not work only for images, but with all other accepted filetypes as well.