Caves should most definately have some forms of wildlife, like you Ruby, I have thought this for awhile. At the moment they are spooky but dull, bats would be an obvious choice and should be found in caves, either as curios or an early food, like rats - but harder to catch, or even aggressive as you reach lower levels.
Also i'd like to see fungi - some edible, some poisonous - the latter could be healed by drinking lots of water maybe.
Moths - common ones could be used as fishing bait in cave pools, with a rare brightly coloured one as a curio.
And finaly - jewels, random and rare to be used in crafting and/or curio's.
I also had some idea's centered around finding cave drawings or ancient skeletons - but with the introduction of the skulls it sort of nullifies them. But, yes, more life in caves pls.
