Honestly, I know it will be difficult for the Devs, but soon we need a complete rework of both water and terrain.
Disclaimer - Some of my ideas may be impossible in the engine or too difficult to program.
Terrain: Has a green-red glow if you're holding any terrain editing tool, you can gauge how close it is to crumbling. When it crumbles, it doesn't just disappear, it actually falls down and if a player is inside/below it they take damage similar to drowning. The tunnels could be supported by stone or wood. All ore veins including ore-like materials would also fall instead of simply breaking or floating.
Water: This would probably be impossible to implement but more realistic water physics. Not just a set water level for all land/ Different levels of water depending on where you dig and not a SET water level. This would require movable water.
These reworks together could create some really interesting underground bases. They would need some sort of enemy and perhaps a whole new cave biome to prevent camping underground during raids. Perhaps smoke could stay indefinitely in caves so you had to dig vents...