I've spent some time in Drakensang Online, trying to get some more ideas for stream stuff, or even just water. What I found instead was a really cool way of building the same shaped tileset over and over again, but with different visuals.
From what I have studied so far, this game uses the following in every tileset:
Terrain A
Terrain B
+1 (both)
+2 (both)
Solid (impassible rock, or a building [universal])
Chasm (universal)
Water (universal)
If you combine these parts, making solid/chasm/water all universal height, as if terrain level 0 will always attach to water the same way, and +1 and +2 just happen to get a water corner, then the total number of unique tiles without duplicates for variation is just over 1027.
Now, you can't portray a good mountain with that, unless you specifically set the height transition higher, or make some specific groups to transition faster, so that is generally incompatible with my granitelands set.
What you can do is something I have wanted to do for quite some time, and that is a dual terrain crypt/cavern with pits, water, and solid space to make that enclosed feel.
Another thing they do with the same tile combos is create a swamp/forest which feels enclosed by trees or chasm.
Their fog falloff is set really low, and they add additional fogging to the Z-axis, so when you look down pits, you don't see very far at all. In terms of NWN distance, their fog falloff on the Z-axis is about 600cm. That is far too low to use in game unless you have a very foggy place. Instead, I'd keep the global fog falloff higher and just add some fog layers to pits. Another alternative would be to have a blanket fog on ALL tiles with a height transition. That might look pretty cool.
I had originally wanted to have really super high places, or seemingly deep, yet transitional, drop-offs in my upcoming underdark sets. What I may do instead is a combo terrain style for each of the three. The karst caves was going to be the first I tinker with, and that was going to be primarily limestone pocket caves, long tunnels, and spherical/dome rooms. That would fit very well with my crypt/cavern combo, and I can keep the required tiles under 2k by omitting the other two height transitions.
Another thing I can do to reduce tile count in a set like that is to restrict transition change to only +1, but create groups which create a +2 transition for me in other ways. That would be far less work, but would not look as good.
For those of you who have played Torchlight 2 or Dungeon Siege 1, you can get an idea of the cave/crypt combos I am planning. You could also take a look at some of the first crypt areas on Drakensang Online.
If you do go view Drakensang, you'll also see that the quantity of tiles for just one crypt using the one tileset is probably about 2-3k, just because of the variants, especially on stairs. They also seem to have a "corridor" style crosser.
Another game I have been examining lately is Dungeons 2, which is an upgrade to Dungeon Keeper.