For the Slideshow option on Desktop Settings / Wallpaper, and the user is using the Random order and a new file appears in one of the wallpaper folders, make that image the next one to show.
This is an unofficial community, wrong place for feature requests. The right place is to my knowledge the bugtracker. This should be in the "Plasma Workspace Wallpapers" section.
For the Slideshow option on Desktop Settings / Wallpaper, and the user is using the Random order and a new file appears in one of the wallpaper folders, make that image the next one to show.