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Activities are great idea, but it's unfortunately not widely used. The idea is to allow users to change this situation. When user create a new activity, allow he/she to apply prefix to XDG_ path (like XDG_CACHE_DIRS, XDG_CONFIG_DIRS, etc. ), so programs will write/read config in additional directories (read from all, but write only to given by user).

If user use such a activity, each started program will inherit environment related to this activity.

I know, there's many bad things, such could happen. First of all, many apps store config/etc. in ~. Also, many apps is wrote to support only one instance and could be broken if user use this feature. Solution could be allowed list, which will contains list of applications, which inherits these special settings. This list could be made by Plasma team, but I thing user should be able to modify it by themselves.