Hey guys, have you thought about diagonall walls? I drew a simple picture, sorry about the lack of drawing skills. Explanation follows.
On the left is how it would appear in the game, on the right are tiles how the pathfinding would see them.
Now obviously the game would need some changes to allow this. First of all, walls would need to be placed in a "click and drag" fashion, not in the "click, click, click" one. Clicking and draging allows you to specify which diagonals are passable and which impassable. I don't know how to explain it better - If you've played The Sims, you know what I'm talking about.
An alternative to clicking and draging would be placing already orientated walls, like placing rails in Transport Tycoon (can't find a screenshot of the rail placing interface, though).
The other significant change is doors. Right now, you have to place doors on an empty square. For my suggestion to work, you would have to build a wall first and then to place the door
on the square with the wall. It would orientate itself as needed.
EDIT: I've just realised that placing walls first would sometimes prevent dwarves from fetching the door from the stockpile
Ok then... place a door on an empty square and then draw a wall through them, at which point they would align as needed.
EDIT2: Better yet, allow both.
EDIT3: Also, if we had support diagonal walls (and hills, and caves, etc...), we wouldn't need to change the grid projection. Even with the standard vertical-horizontal grid, you could build diagonal walls and the nature could generate in diagonal directions, thus allowing for better depth display, while not messing up with the numpad controls.
It is basically octagons on squares.