Not only is it clear that the community wants more underground content, but there is also a very good reason to add more to caves: they are nearly identical to one another, and there is little to look for after you've acquired a large amount of ores. To make underground exploration more fun and rewarding, there should be more cave variants. This is an idea for just one variant. There could be many, many more!
Mushroom Caves
Mushroom caves can be found anywhere in the Overworld with a similar rarity to mine shafts. In mushroom island biomes, they are twice as common. Mushroom caves are only found below y=40.
Appearance
Torches and glowstone would not spawn in this biome. They are in the pictures to make it easier to see.
As you can see, mushroom caves have a floor that's about 50% mycellium, with mushrooms and the occasional somewhat-giant mushroom.
New Mycellium Variant
In this cave, the walls are almost entirely covered with a new variant of mycellium. This variant is white and resembles vines. It grows just like vines, but it's unable to grow if the light level is above 8. As you would expect, you're able to climb it.
When powered, hyphae will transmit the redstone signal to other hyphae. Unlike with redstone, the signal does not decrease over time. However, because hyphae don't actually power anything on their own, you'll have to use a redstone comparator to receive the signal.
Hyphae allow you to transmit redstone signals on walls without losing the signal, but with the drawback that it only works on walls and that you have to read it with a redstone comparator.
Glowing Mushrooms
A new green-tinted white mushroom is occasionally found in the mushroom caves and on the surface of mushroom islands. Glowing mushrooms give off as much light as a torch. They only spread in mushroom island biomes, and even then they spread quite slowly and are unaffected by bone meal, making exploration the main way to obtain them.
You can craft one glowing mushroom into 3 glowing mushroom powder, which can be placed like string. When placed, it gives off the same amount of light as a torch, but it's nearly invisible, just like placed string. This would allow for more hidden lighting.
Introduction
There's no question that a "cave update" or "underground update" is quite popular among the community.
Here are some examples from r/minecraftsuggestions:
https://www.reddit.com/r/minecraftsuggestions/comments/7t822n/its_time_we_put_the_mine_back_into_minecraft/
https://www.reddit.com/r/minecraftsuggestions/comments/cka3d5/biome_specific_caves_and_dungeons/
https://www.reddit.com/r/minecraftsuggestions/comments/6s2l5i/for_a_game_called_minecraft_we_sure_havent_had_a/
https://www.reddit.com/r/minecraftsuggestions/comments/8zebd7/minecraft_really_needs_a_mining_update_here_are_7/
https://www.reddit.com/r/minecraftsuggestions/comments/cnfzwj/new_cave_variant_wet_cave/
Not only is it clear that the community wants more underground content, but there is also a very good reason to add more to caves: they are nearly identical to one another, and there is little to look for after you've acquired a large amount of ores. To make underground exploration more fun and rewarding, there should be more cave variants. This is an idea for just one variant. There could be many, many more!
Mushroom Caves
Mushroom caves can be found anywhere in the Overworld with a similar rarity to mine shafts. In mushroom island biomes, they are twice as common. Mushroom caves are only found below y=40.
Appearance
Torches and glowstone would not spawn in this biome. They are in the pictures to make it easier to see.
As you can see, mushroom caves have a floor that's about 50% mycellium, with mushrooms and the occasional somewhat-giant mushroom.
New Mycellium Variant
In this cave, the walls are almost entirely covered with a new variant of mycellium. This variant is white and resembles vines. It grows just like vines, but it's unable to grow if the light level is above 8. As you would expect, you're able to climb it.
When powered, hyphae will transmit the redstone signal to other hyphae. Unlike with redstone, the signal does not decrease over time. However, because hyphae don't actually power anything on their own, you'll have to use a redstone comparator to receive the signal.
Hyphae allow you to transmit redstone signals on walls without losing the signal, but with the drawback that it only works on walls and that you have to read it with a redstone comparator.
Glowing Mushrooms
A new green-tinted white mushroom is occasionally found in the mushroom caves and on the surface of mushroom islands. Glowing mushrooms give off as much light as a torch. They only spread in mushroom island biomes, and even then they spread quite slowly and are unaffected by bone meal, making exploration the main way to obtain them.
You can craft one glowing mushroom into 3 glowing mushroom powder, which can be placed like string. When placed, it gives off the same amount of light as a torch, but it's nearly invisible, just like placed string. This would allow for more hidden lighting.
Check out my suggestions! Here is one of them: