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Currently, user can configure app switcher to allow to switch between applications. In my opinion, preview of each window of focused application should been displayed on left/right/top/bottom side of switch window panel. After selecting app (by releasing alt and tab), user could search for window or press 0...9 to select window. After some time, effect disappear and default window of selected app will take focus.

Some of those used to exist. At least..close to. Depending on the plasma version you're running, many don't work anymore. Closest you can get is simply use present windows ( maybe change the keybinding if you insist on having alt+tab). Again, depending on the plasma version, stuff like grid layout, parachute and the likes are a thing of the past.