When comes to this idea, I do love Stream Display ads BUT there are some things I feel are really needed to improve this.
I know it's in testing but I feel we should have some more control over them compared to current testing.

Some small things that could work by giving streamers more control, and a basic idea of what I mean that could really improve Stream Display Ads;

1) Make an option so that the only ads that show up on your stream are Stream Display Ads. (people can pick between the two systems?) Ads that don't disrupt the whole stream itself are major and seriously wanted by many
You could go further and add these Stream Display Ads to vods! I know many who watch my own past vods of streams due to timezone differences. This could help with revenue but add on to the idea of these ads being the a great addition to the idea of Stream Display Ads to be a replacement of the old system.
2) Give the streamer the ability to pick how frequently these Stream Display Ads show up on their streams. Much like how we can with the normal pre-rolls/mid-rolls and such.
3) Give the streamer the ability to also add in their own ads for when they are doing sponsored streams. This could be massive for streamers who get sponsors. A way that can be on your guidelines but still be a way for the streamer to have their unique way to show off sponsors