I think a minimalist dirt style would fit this good, because I mean, it looks decent how it is already--perhaps a bit dark, but it really gives it an 'earthy' feel, you know?
I like the grass, although it is slightly repetitious, and it seems to just be 'floating' on the dirt at the moment. If it were my tiles, I'd add roots and such into the dirt, especially if you plan to have trees and stuff, which would not only fix the togetherness problem but also break up the monotony.
The water: I would suggest not dithering the water just yet, because a)water really isn't textured that much, except at the surface maybe, or if it is really grungy water, and b)a thick, flat, saturated body of water, like you've got going already, fits your style. I wouldn't have the grass be in parts where there is water, but instead, dirt, and then seaweed of varying lengths and depths.
I think on the rock, there's the right chunk--the one that looks like a cheerio--that is lit sorta bad...I would reconsider the lighting there. The dark hues could be a bit darker maybe as well...looks kinda flat right now.
That should give you some stuff to think about.
Now you make sure and post that mock-up when it's ready, you hear?!
