Private User Profiles
The concept is not dissimilar to private Tabs except extended into the user profile and then gently broken a little. Each private profile would be password protected and each tab would essentially be a private tab.
The breaking point in this concept is that sessions would optionally be persistent. Such as being able to be logged into Vivaldi Mail, close the browser and then return to the session without the need to re-login. So the 'live' history and cookies of current tabs would be maintained.