It would be great to add a way for the camera to follow a player's mouse.

You'd press a shortcut (for example shift+F2 to follow the 2nd player in the list), and then your camera would pan to always include that player's mouse on the screen. We could still rotate and zoom in/out while watching someone else's mouse and the camera would still pan to follow their mouse. Panning the camera with the edge of the screen, with arrow keys, or with middle mouse button would stop the following.

It might be worth showing a little icon in the top right corner next to your name that shows who's watching your hand, in order to prevent potential cheating cases. It would just be an icon with something like an eye with a hand inside of it, or something.