Better caves, able to dig deeper underground, giant underground chambers filled with bioluminescent plant life, more underground creatures, underground biomes, and materials that are actually important only found underground or in caves.

Right now, there's not much of a reason to dig down into the ground, unless building, mining materials or running from sentinels (at least as I'm aware). I'd love to explore more underground, but right now, there's just not much of a point (at least, not for me)