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Add default panel, activating while D&D. On it, could be inserted folder view for clipboard files. User can move or copy file onto this location, switch window and move onto destination window. For newbie it's hard to use d&d mechanism. Made it simpler!
What else? Actions associated with file extension or other icons, such like send via email, delete, decompress into new directory in clipboard folder, etc. Also add shortcut to open this panel and add new files folder view. Integrated applications could have menu action called fast save. When selected, user do not have to input anything, but app select default name, save into new files folder view and popup this panel, so user could drag icon, where he needs.

I have another solution. You must select one, fear developers/designers.

Instead of panel, made additional plasma overlay window. Widgets would be the same, as i suggested above.

When user start dragging from this overlay window, it disappear. When user move cursor with item onto since screen position, it appear.

Panel and overlay must be configurable. Panel additionally could be placed on center of screen in alternative to normal panels.

Edit: you could also add versioning system to each file in fast save area, with cleanup up disk space application, where user could select other version to delete.
User have trouble with using versioning systems, but versioning systems are very useful. Also add additionally actions to comment file on fast save directory and version comments too.

Additionally, Open File dialog of KF5 could accept D&D request for files. Thanks to that, user could fast save file and move from panel to open file dialog. Of course, better if application window accept D&D events, but we cannot request each application to integrate with this idea.