Allow user to create activity as special and attach shortcut, plasmoid or window mapping to it.
This activities will be hidden by default, but user may associate keyboard shortcut/hot corner to shown it up.
Also, they may use kwin to press special button on window called shown activity related to this window. Activity/Window mapping could been set inside special window/app options under kwin. They could set the same rules to match window or app like normal, so window title, window class, app name, etc.
Plasmoid, placed on panel, for example, could shown up this activity.
What do you think? To what use this? For example to takes notes on game window. Simply do mapping game window → activity, press button on window title and make notes!
Additionally, may use plasmoid placed on panel to shown up panel like in Windows 10. There could been system status, notes, etc.
Hot corners could been used like in plasmoid showing activity, but additionally it could be great if plasma moves some effects (like mission control or whatever) into plasmoid and user could create custom mission controls by placing plasmoids.
One think: This would not been normal activity, because activity has associated thinks like files, windows, etc. This would be rather plasma window with plasmoids, but associating file lists with this special kind of activity could been great. Also, associating window is not bad idea, I think, but none of window could been placed only on special activities (it should been placed also on normal activity), for example to filter out window list delivered to plasmoids, so user could place task list into special activity.