What do you thing about isolating of cookie visibility between private windows?
Currently, all cookies in the profile are visible by all profile windows, either normal or private. Therefore, if you open a private window, log in to a site, then open another private window, and open the same site, it will identify you by the cookie accepted by the first private window. The only way to open the site anonymously is to close first private window, or create a separate profile.
If there was an option to isolate cookie visibility, each private window would have its own temporary cookie set. Login sessions would be local within a private window, not common for all profile windows as is currently doing. But different tabs in a private window will still share common cookies, allowing to access different session pages at the same time.