Filling spaces between walls

It would be nice to have a texture that fills the space between walls, dynamically adjusting to the required space. i.e. both in terms of area and shape.

Water already works in this way but only fills up to ground level. The proposed 'space filler' would do a similar job, but up to the height of the lowest wall in the space to be filled.

The purpose of this would be to fill all those void spaces that serve no function on the map.

Though similar, this is NOT the same as thicker walls.