I am a huge stickler for immersive and ambient sounds in games; it helps sell the whole experience. A great example of what I mean is The-Between-Lands mod for Minecraft. There's a symphony of noises and animal sounds that brings you to that place and surrounds you.
I don't know if some of these are already planned/implemented.
Some specific things could be:
crickets/cicadas chirping
frogs bellowing
bugs in swamps
birds songs
water dipping in caves
echoes in caves
sticks cracking when you step on them
wind rushing over mountains
larger wave sounds for larger water bodies
trees creaking in the wind
rattlesnakes in desserts
loose pebbles falling in caves/mountains
etc.
Basically whatever animals aren't added into the game, make them into environment sounds, and more auditory cues from the diverse landscape.
loving the game so far! :]