I believe Minecraft caves and the underground need an update/revamp similar in scale to the aquatic update to add more depth to the game literally and figuratively. Here are some of my ideas for how this update could be done:
Make Underground Deeper: The underground in Minecraft should be made a lot deeper. For example, if you are standing on the surface in a plains biome, you are likely standing around Y 65. I think this should be at least doubled to around 130. This increased space would better allow for a variety of things that are listed below. At lower elevations, blocks that take longer to mine (like hardened stone, compressed dirt, etc) could be added to make lower elevations more difficult and would push players to gather better resources before traveling deeper underground.
New Ores: Current ores in the game can remain spawning at their current Y levels. New ores could be added such as copper, tin, amethyst, etc which would all be used to make armor, weapons, tools, etc. These materials should all be weaker than Diamond. Amethyst could be somewhere inbetween diamond and iron in strength while copper and tin would be weaker than iron. Copper could be the weakest and spawn anywhere from Y 128 to Y 0 while tin would spawn anywhere from Y 96 to Y 0. I think adding new ores would benefit the game because right now, starting a new world generally goes as follows: You gather some quick resources and go mining and get full iron armor. You then have that iron armor until you can get diamond armor. I think adding new steps with new ores would add some more depth. Especially since iron armor is quite powerful for how easy it is to obtain.
New Structures: Dungeons could be revamped to instead be larger underground structures with more loot and some traps. Cave villages could be added (using special cave villagers which have unique caving trades). Abandoned cities of an ancient enderman worshipping cult could be added (credit to YouTuber Whitelight for that idea) This would help make the underground more exciting.
New Mobs: New mobs could be added to make the game more challenging. The lower the elevation, the more challenging the mobs. Miner zombies could be added that have more health and deal more damage, cave creepers that blend in more with stone could have a higher blast radius, an entirely new cyclops/ogre mob could added which lobs rocks and deals melee damage as well, or a mob that can swim in lava and set the player on fire if it comes into contact with you could be at the lowest elevations, or any other mobs that Mojang could think of to add.
Cave Biomes: These cave biomes would depend on the biome at the surface. For example, under snow biomes could be ice caves full of different kinds of ice and cold stone could be under snow biomes, while under a jungle could be overgrowth caves full of leaves, grass, vines, etc. This would help bring more variation underground.
These changes in my opinion would make mining and the game as a whole a lot more interesting. Mining would be more rewarding as it is harder to get more valuable materials that are at lower elevations. It would also make the game and caving specifically more exciting with the new mobs, ores, cave biomes, and structures.
I do think a cave update is a good idea, just not in some of the ways you're proposing.
I do agree that making the underground bigger is a great idea. However, I think that no matter what, diamond should stay at Y 16 and below. I also think that iron should generate anywhere from our current Y 30ish to about our Y -10, and then not generate any more (as to not clog up the new space with iron instead of new ores).
I am down for pretty much any type of ore imaginable, as long at least one of them has a special use beside tools and armor. I also don't want too many more ores, but also enough to be worthy of a full update status. Maybe 3 or 4 new ones. I think diamond should stay strongest, a new ore between iron and diamond is added, iron is third best, and two or three new ores are worse than iron.
I think cave biomes and new cave mobs are a great idea, but i think dungeons and abandoned mineshafts are fine for now,.
Overall I agree with you, I just think that diamond should stay at current Y 10 when worse ores generate below it, and that structures don't need to be updated just yet.
Just to clarify: I think all current ores should spawn at their current y values and bedrock should remain at about y 0. What I'm saying is the whole underground area should be about at least double what it is now. Diamonds would still be at the very bottom anywhere from y 1-16. However the average elevation of a player standing on the surface would be around y 130. I should add with my ideas that the max build height should be at least y 512 instead of the current y 256.