First off, I know water is being worked on etc...
So, imagine when the world is generated, it also generates pockets of fluid underground. Water, oil, natural gas (gas pocket!), lava, and maybe some fantasy element like a cavern with an ooze lake where specific monster lives?
- source of underground water, and maybe a reason to have pumps in the game so you can get that water to the surface?
- a way to introduce something like oil and natural gas instead of just a pool on the surface or whatever
- increased danger while mining
- interesting building opportunities
- increased monster ideas