This probably wont be a popular idea for a feature but I think it's pretty neat.
I would like to be able to play some of my films in black and white mode(grayscale); it could be enabled in the settings before/during playback. I have done this using the videoplayer called Videolan(VLC), and the results are usually incredible. Here are a few example of films where this is worth trying:
A Clockwork Orange (1971)
The Godfather (1972)
The Raiders of the Lost Ark (1981)
Blade Runner (1982)
The Crow (1994)
