Here's another thing. Couldn't you actually light tiles based on their proximity to light sources? Outdoors = full bright, indoors less so. If a wall is engraved, maybe it could be considered to hold a sconce, thus lighting tiles around it.
In theory we can do all kinds of funky things, but there is no way to tell if a tile is outdoors from just tile number and colour :F
Yeah, probably, I was thinking about what tiles are always outdoors and whether they could check for those, or if a tile has a roof then it's obviously indoors. But no matter, that's probably A Kludge Too Far.
Still.. what about the idea of light sources? I mean it would increase the number of tiles directly, but for starters a simple photoshop action could apply a basic 'beam' of light overspill to tile edges. Then say if a tile is near a furnace, it gets replaced by a tile with light falling from the appropriate compass direction.
..if a floor is smoothed, and runs up against a wall, then the pattern could embellish around the edge. Hell, you could do away with tiled patterns altogether, so that single polished floors on their own worked okay, but when placed next to others, the patterns meshed.
Don't know if objects count as tiles for these purposes but you could have double beds for married dwarves, even.
The possibilites for interesting geological features are pretty awesome too, like gem formations that properly mesh and change as they are gradually dug out from the sides.