Trees would be nice... but personally I can think of a few things I would put higher up the list, the things that sort of break the disbelief when you see them in two places standing for different things. Oh, and more rock tiles - obviously a lot of rock types look the same, but a lot don't look like anything else and should probably have their own tile.
Loving the latest release though - seeing the dig designations makes things so much easier.
Personally I've made a few alterations.. I've added iconographic style hashed lines and dots to the 3 dig-animation tiles, since I don't really dig (hur) the way that when they bash a rock, it suddenly transforms into a different rock-style and fades out (I've left that pattern in the background underneath the lines and dots and it looks really awesome though).
In game:
In case anyone wants, here it is:
Also, I increased the contrast to all the rock types. Not sure if that will screw the transparency up yet, haven't tested it. They looked a bit light to my eyes (trade off for visible designations tiles I guess - the old darker tiles looked more 'rocky' and almost perfect).
Also, replaced the stockpile tile. Your tile makes my eyes hurt! This is one I created for a personal set I was using previously. I thought about what the dwarves would use to lay down a stockpile, surely they would not go to the effort of paving an area in neat tiles? So.. they use planks of wood!
Edited this one up for 18X18.. and it tiles quite nicely, I think! Here it is in case anyone wants:
stockpile:
tiled a lot:
in game:
Not as pretty as your nice anti-aliased floor tiles but best I could do
You can see how marble (white rock) looks with the replaced dig tile (it shares the third one), and also I added a bevel to the 'solid colour' tile, something Mayday does on his set because it makes the Trade Depot look f'awesome
Anyway sorry for mangling your nice pixel art, just some ideas I thought I'd share in case any appeal!