Underground: Caverns

I think that there should be three volumes of caverns that form: Huge, Large and Room.

Huge should be impressive; 100-500 blocks in diameter in randomly shaped blob.

It should not tied to any specific biome, but have a larger chance to form depending on the 'thickness' of the land.

It would be the rarest volume to form and have a chance to be filled with some random internals:

• Some of it open to the sky and the whole floor filled with a random biome
• Filled with stalagtite/mites and columns
• Floor covered with stepped pools
• Floor is one large lake (with waterfalls)
• Giant mushroom biome (Phosphorescent)
• Dead trees (all stripped, no foliage)
• Bare with a huge skeleton of a beast
• An abandoned village
• A ruined village
• A ruined fortress
• A pirate ship

Large spaces would hold curios; 50-100 blocks diameter

The floor would have an edge that dropped into a crevice with the center of the space raised. There would be a random 'access' to the center.

The shape of the space of a large cavern would be random:

• squared (not a cube) with carved pillars,
• perfectly domed
• random/natural dome
• egg/spire 

Random features that would generate in the middle of the large space:

• spawner
• shrine
• camp
• ship wreck
• lava/water spout to a random height 
• abandoned/incomplete nether portal
• large oak tree
• hut/building
• hole/stairs down
• central spire with ladder up to the top
• upside down tree from the ceiling
• cage/holding cell

The access to the central feature within the large cavern would be granted via a random method:

• A boat and piers
• wooden bridge
• worked stone bridge
• natural stone walkway
• broken fences
• stepping stones
• high walkway
• pattern on floor (both sides)
• mine rail & cart

Room spaces would be carved out pockets; 10-50 blocks in diameter, not specifically linked to anything other than the normal cavern system.

• Fortress grand hall
• Fortress throne room
• Fortress arena
• Doorway
• Smuggler's store
• Crystal cavern (coloured glass blocks with light behind them, specifically in the ceiling)
• Safehouse
• Tomb
• Structured mushroom farm (raised beds)
• Alter
• Mansion library
• Smith