If only remuxing could get rid of the black bars that Blu-Rays include in the video stream because the spec only allows a small number of resolutions which surprisingly don't include any 21:10 ish aspect ratio common with movies.

Even just a convention for storing crop coordinates in metadata that players could automatically make use of would be godsent. Although a perfect solution would need to support varying this throughout the movie as some cunts (Nolan) like to use different aspect ratios for diffent shots.