SketchUp Developers Observer Wishlist

Google SketchUp software engineers are asking the developer community to provide feedback about the Sketchup Observer classes.

In order to try to prioritize which Observer to fix first I would love to collect some data on the following topics:

what are the Observers that you would like to see added to SketchUp?

which are the observers that you would like to use but that do not work?

When I have some data I am hoping to push for fixing/adding those.

The discussion can be read here.