When Toady does start work on the graphics in DF I'd like to see more context sensitive tiles which respond to the tiles around them like you suggest. There are plenty of things already in game which might be candidates for this. The one that immediately jumps to mind are veins of ore, there could be a number of different ore tiles shared between the various ores but coloured differently for each.
I did a quick animated mock-up. Obviously others could do a better job especially with the tile used for the ore.
The scraggly edges of the vein could be stored as a greyscale mask and could be combined with the texture of the surrounding rock and the ore on the fly so you wouldn't need to make new ones for each ore / rock combination.
On a similar note someone, in another topic, mentioned transitional tiles and posted the following image:
Other places where this technique might be appropriate include the boundaries between different rock types. Also digging out a room could leave the walls slightly uneven until smoothed. Smoothed walls could be more context sensitive than they are now recognising which side has been smoothed and what direction they face, knowing they are in a corner or at a junction and what junction would give more freedom to make interesting tilesets.
Something simple I threw together, not really appropriate for DF as those walls are all two tiles thick but again there are only three types of tile in the picture, un-dug, smoothed-wall and floor but the smoothed-wall is represented a number of different context sensitive ways.