I have thought about using sub collections and have suggested that to others that asked me about the lack of a playlist menu. The only downside to that is if you were to use it for a decent number of playlists that can amount to a ton of collections. If you were to use playlists by genres for example, you could easily have 20+ playlists/collections per system. Doing that for a good amount of systems would result in hundreds of collections.
The media issue is where it starts to get complicated. I would be perfectly happy with having a global directory for playlist logos in the collections/_common directory. However, I'm sure some people would want different logos per collection. Another potential speed bump would be the layout of the playlist menu. If it were to use the top collection menu similar to subcollections the lack of video or other layout elements like story or info wouldn't look great, such as the Aeon Nox theme. So at that point it would almost require it's own layout or even a global playlist collection (I told you it would get complicated
).
That's just what I've thought about in regards to this potential feature, and I'm open to what others may think about the implementation.