These are all (possible!) ideas that could be implemented to improve the state of the Plasma Workspaces by using KWin's compositing technologies more aggresively and creatively.
We'll need to keep the existing fallbacks in place to preserve the non-composited case. This would augment those fallbacks.
If the wallpaper is redirectable (rather than painted on the Containment itself), then the DesktopView can be used as the DashboardView, fading between wallpapers can be moved out of the wallpaper plugins, wallpapers won't need to repaint themselves just because widgets move over them, etc.
Right now tooltips are normal windows, this makes moving and resizing them rather inefficient. if kwin would directly render the tooltip texture on screen, it would be possible to have smooth resize, move and fade of tooltips on screen
Probably a new effect in kwin is needed to provide the tooltips. It could use an EffectFrame for the background and could use the normal TaskbarThumbnail effect to render windows on top of it. More hooks in KWin are needed to set Icon and text. Probably best is if KWin uses the existing thumbnail layouting code and renders to a texture. When something changes KWin could render to a new texture and crossfade between them.
Best moved into KWin. KWin already has the code for it, it just needs to be refactored and made more generic. We need to define what Plasma needs from KWin and whether it makes sense to have a more generic screen edge handling (e.g. triggering anything what is available to DBus)
Would allow KWin to stack the shadow correctly. All the required features listed in this page show that KWin needs to integrate textures/effect frames into the stacking order on compositing. This needs some refactoring (maybe GSoC project). Fast solutions are nevertheless possible.