I think a new block type that could be made is silt. It forms naturally between sand-dirt or sand-grass boundaries. Cacti or trees do not grow naturally on it, but you can plant them. It is like a sand-dirt hybrid. Also, it does not have gravity like sand does. It would look sort of like dirt, but with lighter specks in it. Here is a picture I made in paint that looks like what I am thinking of.
+1 diamond for you mister!