Quick Sand would be a ground that you would sink through. We have a few of this but it would be good if it became an official ground in editor.

I know some people use spider web module to create quicksand already, though to have quicksand as a ground would be quite interesting in my opinion. :P

There is actually a way to create quicksand.
Anyways, I just made the quicksand and it really is easy.
http://pastebin.com/biASYCQ5
While it sounds feasible for an Official Ground in Map Editor, you have to consider why they rejected the Cactus Ground.
If it can be recreated easily in Map Editor then there is no point in adding a special ground.