Empty space can have a custom graphic, or not ? (yes)
Why not have more empty space graphics ?
So that could be EC (empty customs) that behaves exactly like empty space, only with the difference, that they have other graphics (and sounds).
If that would be implented, 16 + 16 would be enough...

I have some other especial good future plans for that too...
2) If you give the normal empty space graphics, like these from a grid for example, and something moves, the empty space´s graphics seem really bad if some small objects are moving through it...
Maybe the black tile of the element, could be transparent, or not ? I mean like the player is "over" a CE, you can see the CE and the player over it.
EDIT: I see you have fixed that already since this last version, didn´t you ?
