Feature enabling the ability to "zoom-in" on PC, similar to pinch-to-zoom.
This would be a nice accessibility feature to include on PC for users with worsened eyesight. It would also be beneficial to users with limited monitor space (who also want to have the chat up) or who are watching multiple streams at a time and may want to adjust their zoom level to improve their view.
The benefit of this feature would likely also extend to Twitch in general, as it creates an opportunity to delight users by granting them a little more control over their UI.
The greatest con of this feature would be if the zoom would be used irresponsibly or inappropriately. Therefore, reasonable limitations of zoom would be necessary to prevent misuse. Even a small bit of zoom (5-20%) could greatly benefit users with eyesight issues and also provide users some utility to create their ideal view.
While theatre mode, full screen, and picture-in-picture mode provide users some viewing options, this addition would better allow users to adjust their view to their preference.