Though worded like a suggestion, it's none, because it'd be a big change that would break a few systems. It's more wondering why Toady implemented 3d the way he did. I'm going to elaborate on my thoughts and am curious about yours.
In DF, one spacial direction is treaded differently from the others. Tiles on adjacent z-levels are separated by floor tiles, while adjacent tiles in x- and y-direction aren't separated by some kind of 'wall tile'.
An opposing system, without floor tiles, would be: Two levels of a fort are separated by an unmined z-level. Equivalent to how two rooms of a fort are seperated by an unmined row of tiles.
Though trivial at first, this would make fort geometry and constructions easier to understand, because they'd be governed by the same rules in all spatial directions.
Examples in current DF:
Doors inhibit one tile, but floor hatches don't. Thus doors can be blocked by units, floor hatches can't.
Mining reveals the materials of adjacent tiles in x- and y-direction, but not the materials of tiles above and below.
Diagonal movement is possible on one z-level, but not over z-levels.
Unit can travel from 1 to 2:
####
1+##
##+2
####
(Flying) unit can't travel from 3 to 4:
z+1
####
3+##
####
z
####
##+4
####
Even though side view suggests identical geometry:
####
z+1 3+##
z ##+4
####
Legend:
# Wall
+ Floor
This came to mind while thinking about 3d mineral veins. How are we going to follow an ore vein across z-levels, when the materials above and below aren't revealed. I'd like for something that looks as follows: