I'd like to see Minecraft dive a bit deeper into the ambience. I've played with a couple of different mods which adds a lot of tiny details here and there which doesn't necessary improve the gameplay, but the aesthetic and overall feeling gets a boost!
For example: More sounds. The blow of the wind across plains and through the trees. Different type of bird sounds calling in the distance. A tiny fly buzzing about. Minecraft can be very quiet if you're not doing something very particular.
Also more variety of foliage. If you're familiar with the BetterFoliage mod, imagine some sort of tweak like that. Lusher trees, a bit ''bushier'' grass, reeds in shallow water, tiny leaves falling slowly from the trees etc.
