Make Twitch player fullscreen, and put an optional chat on the side for devices with resolutions above 1920x1080.
The mobile device that I mainly stream Twitch on is a OnePlus 5T. This device's resolution is 2160x1080, at an 18:9 aspect ratio. If the player were to stay at a side of the display at a 1920x1080 resolution, and the chat at the other side (at a resolution of 240x1080), it would lead to a much more interactive experience. The chat would also be optional, and the viewer would be centered in the middle of the screen at a resolution of 1920x1080, similar to how it works right now if chat were to be toggled off of its fullscreen 240x1080 mode. A concern would be that this would only work on devices such as mine, with an aspect ratio above 16:9, but it would be a feature that would be highly beneficial for a phone like the 5T.