With as common as different aspect ratios are getting, it'd be nice if ratios other than 16:9 were supported by the player on the site. This would let the growing number of ultrawide screens have more utilization.
Well other aspect ratios are supported TBH.
For example I stream at 21:9 aspect ration and the twitch player shows it accordingly.
When in fullscreen it shows the stream as expected, altho when in player mode there are black bars top and bottom.