There is a Landscape Texture Replacer mod in the work. Its creator, redwoodtreesprite, released the dirt and swamp water textures (shown in the current Picture of the Day) as a standalone download at the Round Table's Modcrafting Guild:

"Some are completely new, and others are the game textures tweaked and sharpened by me. All of the textures are saved in the way Nival did, and have the same file size, so other than extra room taken in your data files, there should be no effect on game performance."

The release notes include installation instructions, credits for each component of the textures and requests if you plan to reuse or redistribute the mod. Some really nice work.