It's important to think about the underground content, as there's a lot of untapped potential in it. It may be cool to have custom generated caves with differing themes underground. The default theme would include the regular deceased blocks, but several variants would also be generated (such as spider nests, soul sand caves, and undead tunnels).

I have a Perlin-noise based cave generator that may be worth looking into. One of the primary benefits it offers is unending caves-- you can walk for an eternity and never find a dead end. If you connected dungeon entrances to the caves and provided different cave difficulty levels, we could have an entirely separate group of content below the ground.

Possible cave features include:

• quick dropoffs to lower levels
• large, open chasms with underground decor
• bubbling witch water lakes deep underground
• a cave boss, such as a giant spider in a spider nest
• another material/ore found at much deeper levels than Spookium
• entrances to dungeons and other custom cave structures

We could also have cave entrances located at the top of the world. You would find pits into the ground at random locations, which drop you into the nearest cave. There could be special items located deep in the cave which provide a method of escaping or teleporting back to spawn.