In reply to Peter Shaw.

I want to answer this question:

Stricter guidelines for plugins.
Plugin notifications only on the respective plugin screen.

At the end of the day you can't control commercial plugins or non-repo plugins and they are free to inject ads and notifications wherever they want.

But for the official plugin repository, see my 2 points above.

I do agree that the wp-notify project is an improvement and I'd like to see a nice bell icon somewhere top right of the screen where all the messages are.