How about if some little things would become more controllable and customizable for the viewer. In streams (of course if the streamer would turn on this feature) you would be able to control more of what you see on your screen. For example, facecams. Depending on the game facecams can be a bit on the way of the gameplay. Streamers do their best to position it in the best positon and size possible but what if that was in the viewers hands and it would be even easier for the streamer. The streamer would be able to record his/her face just like usual, but doesn't have to indicate it to any fixed position. The facecam would be a toggleable feature for the viewer and could be moved and sized up or down freely on your own screen only for yourself. It could act like a little window that you just move around, if you want. Or toggle totally off, if you feel like it's only blocking your screen, or even move it to another monitor if you own one.

In addition to this kind of interactivity some more toggleable features would come on. If you like an on screen chat you could toggle and size that on your liking. Even little mini polls, tiny "kahoot like" games, and other could be toggled on, hosted by the streamer and controlled by the viewer. There allready are polls on streams but what if there were more interactive ones, like kahoot style activities.