Well, there are already tools for showing a pp counter, so I don't think it needs to be implemented in the current client (maybe osu!lazer but idk). Instead of just a pp counter, it could do something like showing UI elements to the viewers but hiding it for the streamer so that the gameplay experience is good for the streamer and the viewing experience for the viewers is good without the streamer compensating. e.g. the key overlay is visible to the viewers but not the streamer, showing the storyboard/video at 75% opacity for the viewers but not for the streamer. This is not limited to UI as well, e.g. audio is set to 30% master-50% music-100% hitsounds for the streamer and the viewers it's 100%-30%-100%, you get the idea.
Basically makes 2 "profiles" of options, one affecting the streamer and one affecting the viewers. Then again, coding this is quite complicated (at least for the current client) having to implement YouTube and Twitch encoder support so that these options would work (probably, I only have a really limited knowledge when it comes to streaming).
It's a good idea with a lot of potential, but there are a lot of complications if it gets implemented. Maybe on osu!lazer it will get implemented with enough support but idk :/