I would love to be able to make the background in the letterbox scale mode transparent. The reason is that I really like how the letterbox scale mode centres the game on the screen.
Letterbox scale is the best option IMO. However, if you could make the background transparent, you could see the objects around the viewport.
Sizing your game is always one of the hardest things to do as a developer.
It would also be great if we could access the background color programmatically.
I have been prototyping some games and I think if these two features were added, it would make the gaming experience much better.