Rocks'n'Diamonds

Set a CE as "empty space" so that creatures can move onto it.
This is complicated, and would probably require making a second "out field" matrix representing the entire level for elements that are being walked upon by other elements.
Alternately, you could just make the element disappear temporarily and watch the row,col that is occupied until it is free, then reappear?

1. This has been mentioned a couple of times before (not sure where though, could be on an old forum)

2. This is (somewhat) possible using CEs that can "dig" other elements and change them to what they previously where.

Look in the CE config page 2, there you'll see things about CE can dig <element> and can leave behind / change it to <element>.
If a CE eats CE2 and change it to CE2, then it will look like CE2 just doesn't exists.

Even though this means that you need a CE for almost each moveable/fallable element in your level, it's still possible. Since almost all elements can be converted into CEs.

Elements that are "walkable" for other elements are generally a problem, as this would be a new concept that is not possible with the current engine. As this would require major engine changes, it's not likely to be added before the upcoming implementation of the three native game engines.