Whenever I mine, I just see cave systems and ores and just abandoned mines and the occasional fossil. So here are my ideas.
GIANT CAVE SYSTEMS - So usually cave systems are narrow and the like. However there are things a giant cave system has.
STALAGMITES AND STALACTITES - Stalagmites and stalactites look like ice spikes except that they are made of stone and occasionally, ores.
Mining Hub - These are rare structures which have villager miners and blacksmith. They look like abandoned mines except that its not actually abandoned and there are glass, torches, and villagers in it. Also there are underground houses.
The Mantle - There will be tons of lava and magma right above the beddrock. However, more ores will spawn here, but it's so dangerous.
The first one used to exist until Mojang nerfed caves in 1.7 by making cave systems much smaller; for example:
1.6.4 (back to InfDev):
Here are some screenshots from cave systems in 1.6.4 - they are often so dense that individual tunnels merge together into huge ragged caverns:
That said, imagine finding caves like these:
These are taken from the middle of the same ravine, looking towards the ends on a render distance of 8; it was more than 300 blocks long, meaning that you'd need a render distance of 19+ to see from one end to the other:
Aside from these giant caves there are also many variants of caves and cave systems:
Other things that could be added include underground biomes, which have stone/dirt/gravel replaced by other blocks, like sandstone in deserts (this can be seen in some of the screenshots above; I've since extended it all the way down to bedrock), with ores to match (I use a transparent texture that contains the actual ore bits which is overlayed on top of the base block; this way you only need to make one new texture per ore):
Jungle; the grass is a special variant which ignores light (1.7+ has a bug where plants do not uproot and can generate in low light levels, causing grass and flowers to generate in dark caves):
This is a special variant of cave which is filled with jungle vegetation:
Desert; you can see ores with the sandstone texture as well as sandstone stalagmites/stalactites:
Mesa; unlike 1.10+ husks and strays can spawn anywhere, not just at the surface (pointless when you spend all your time caving), and have additional variants to match their biomes:
Badlands (distinct from mesa in TMCW); you can also see some veins of red clay (a recolored version of the normal gray clay):
Swamp; 50% of udnerground dirt veins are clay instead:
Volcanic wasteland; regular stone and andesite are reversed so you have pockets of stone in andesite instead of the other way around; there are also veins of magma blocks and more lava springs and pools, and no water at all:
Iceland, a mesa-like biome made out of packed ice and its variants:
A colossal cave and ravine under a desert; you can also see the additional colors and variants of mushrooms that I added:
Even the Nether has been improved (a dimension that I ignored until recently since I barely spend any time in it):
Nether dungeons (only normal skeletons can spawn from them), as well as Nether gold ore:
There are other ways that caving could be improved as well; for example, imagine if you could make maps of explored caves in-game (these only show caves that are lit up by torches placed by the player; they also work in the Nether):
Also, 1.7 made dungeons and mineshafts much rarer; while the latter are a bit too common anyway they could be improved by ensuring that they don't generate on top of one another, as well as have different types of wood depending on the biome (e.g. oak forests have oak wood (the default wood type), spruce forests spruce wood, and so on), and vary more in size:
A spruce mineshaft; the center room and staircases also generate wooden platforms over air (mineshafts have been broken in vanilla since 1.10 as they no longer generate floating platforms in most cases; see MC-103559 and MC-105035):
A birch mineshaft in a quartz desert (chosen as it is a lighter shade than oak, otherwise biomes without trees usually have oak. Redstone torches are also used for lighting so they don't interfere with mob spawning or show up on cave maps):
A jungle mineshaft (in all cases the wood type is based on the most common biome within a 5x5 chunk area around the center):
Dungeons also vary more, including a larger variant with two spawners and more types of mobs and blocks used for the walls (notably, they make chiseled stone bricks much more common as otherwise they are uncraftable in 1.6.4 and are only found in jungle temples), which spawn much faster as well (spawners were nerfed though so that the mobs spawned wills top dropping loot and XP if they spawn too many; I'm very biased against farming, especially autofarms, since I think it ruins the spirit of the game):
Mining Hub [/b]- These are rare structures which have villager miners and blacksmith. They look like abandoned mines except that its not actually abandoned and there are glass, torches, and villagers in it. Also there are underground houses.
This sounds SO cool. I'll love to be mining and BANG, villagers. It is a cool idea indeed.
TheMasterCaver gave great examples about a giant cave systems. Those kind of things would be awesome for both exploring and building.
I’d love for their to be biomes and villagers that you could find underground! At one point, Mojang mentioned adding crystal caves to the underground toy could find, but they haven’t put them in yet. The crystals as they described it had no real use except for decoration. But yeah, underground biomes would make mining so fun!
The Meaning of Life, the Universe, and Everything.
There used to be so called caves in older versions of MC. But for some reason they were phased out in later versions. Luckily TheMasterCaver brought this up again with pictures (liked that post back then too, lurking in the shadows before joining the forum)... so you can live with the thought that a lot of people would like to have more 'Mine' to the name Minecraft and thus caverns.^^
I made a post this like a month ago, and it got hate, but your idea seems to fit some of mine. Nonetheless, I like yours.
An underground biome.
I think there should be large pockets like in the picture above. I also think Giant Mushrooms should be able to generate in the underground biomes, giving off bioluminescence.
More stone types.
About the large biomes, Mushroom and Swamp should be the most common and unique, with new plants, gouls, ect...
Even small caves should be dependant on the biomes, Dark Oak and Jungle caves should have roots hanging from the top, and Mesa caves should be made of clay.
Desert caves would be made of sandstone. Of course though, caves made of stone would still be a thing, and be the most common. I made a post on something called Cave Fog, I can't explain it, but the post is here.
I would love underground forests like under jungles and glowing mushroom caves under mushroom island biomes. And maybe add sink holes with light from the surface coming through crystal caves and lava tubes limestone caves underground rivers. Infested slime caves. ice caves magma caves oil deposits fossil caves and underground villages.