Forcing screen resolution / changing aspect ratio is what I'd want. I've got my Chromecast connected to an ancient 4:3 TV and I'd love the Chromecast to support that, or to at least not pillarbox 4:3 Youtube content. Unfortunately there's no setting on my TV to change the aspect ratio, I have to do it on my devices.

(Yes, I know, I should just upgrade the TV, but in that case I probably wouldn't have bought the Chromecast.)