I'd like to see pits for the caves as well as the wooden walkways like in the frontier tileset (for both water and pits).

Maybe for the larger buildings you can use the unused isometric building tiles originally intended for Flare. I'd love to see those worked into the grasslands tileset.

I'd like to see tables, chairs, and other furniture for the dungeon tilesets. More varied (and larger) statues for the dungeons. The other thing for dungeons is special wall tiles with art for arrow/dart/fireball traps.

I think the dungeon and cave tiles sets will benefit greatly with the addition of up and down stairs. The added entrances and exits would also add a nice polishing touch.
I really like the "up / down" stairs from Diablo 1 "beneath the crypt" ...I think I remember correctly.... >>

Maybe for the cave tile set the bones / debris could be separate from the actual floor tiles. This way the bones / debris could be used more flexibly with different floor tiles, and possibly other tile sets.
For the cave tile set I think some crystals would be nice, possibly a small jagged outcrop of crystals extending upwards at slightly different angles. Could be made at different sizes and colors. There are some "mine" type tiles with tracks and carts, so some crystals could provide a reason for them? ...someone could even make a NOXish type game with them :)
Also a few vines could possibly be added to the cave wall tiles.
The reasoning behind my suggestions for the cave tile set is due to the lack of color or more correctly, lack of differentiation. The cave tile set does not have a lot of color, which is fine for a single cave, but if you have multiple caves, maybe add a few tiles of to differentiate the caves, top level with vines, bottom with blue crystals, or different caves with different themes, or just to add variety and a more interesting environment.
Half height cave walls?

To add "dirty" to the dungeon tile set, you could add some slime oozing from walls? or some gunky pipes with steam or sludge?
You could add some torture devices as eye candy to the dungeon tile set.
Some doors between dungeon rooms could be useful.

For the grassland tile sets a few lush trees (they don't have to be cheerful bright green) , not just dead trees or trees with a few leaves on them, but with thick twisted trunks and a full head of leafy branches? A larger variety of plant and bushes, maybe tall grass...

A nice idea for a tile set could be a futuristic one like from the vaults in Fallout 2....(yes, I know its not for this game, just throwing an idea out there for future games)

While not all of my ideas are appropriate, I thought I would just add them in case they helped generate other ideas.

I can only add Spider Anillon webs to the list.

Maybe a few other blocking terrains (thorny bushes, collapsed caves)

Cool improvments could be made with more buildings, maybe a wall and stuff. Currently there is an outpost but only limited way to create a City some buildings like marketplace(bazar) oder weaponsmith , church, huge castle and stuffs would be very cool maybe battle arena or training ground with training dummys and stuffs