I have heard a lot of people say that the next update should be an "Underground Update" of sorts, where they revamp the underground just like how they revamped the oceans in the Update Aquatic.
Rather than list off every new thing I'd like added to the underground like new cave types, new ores, new structures, new mobs... I'm going to suggest changes to the distribution of ores within biomes.
Currently, between each biome if you go to an extreme hills biome you can find emerald ores, and if you go to a mesa/badlands biome you can find loads of gold at layers 32 and up. This makes mining in these two biomes more exciting than other biomes, and I think if they implemented differing ore generations between existing biomes it would make mining in different biomes for different resources that may be more common in that biome a thing.
These are my rough thoughts of how ore generation should be changed for each biome. Feel free to comment other any other changes you can think of:
(Note: I'm using the 1.13 names of biomes)
Forest, Birch Forest, Plains:
Stays the same, so these biomes are like a 'baseline'.
Slightly increased gold.
Slightly increased lapis lazuli.
Dark Forest, Swamp:
Has much more coal at layers 32 and above. Overall iron, gold, diamonds, lapis lazuli, and redstone decreased slightly.
More iron. Gold and coal the same, but decreased diamonds, lapis lazuli and redstone.
Can generate with iron across all levels (at the same rate as normal), making mining into shattered savanas and plateaus worthwhile. Coal and gold reduced slightly. Diamonds, lapis lazuli and redstone the same.
As before, loads of gold at layers 32 and above. To balance this coal and iron should be greatly reduced across all levels, and diamonds and lapis lazuli slightly decreased.
Has more coal across all levels. Emeralds can be found here at the same rate and levels as extreme hills biomes. Diamonds can also be found up to layer 40, the diamonds being found at layers 16-40 generating in a similar manner to emerald ore but at twice the rarity. However, iron, gold, lapis lazuli and redstone are reduced.
Mountains and Wooded Mountains:
As before, Emeralds can be found up to layer 32.
Emeralds can be found up to layer 32. Redstone and lapis lazuli can be found above layer 80. However, iron and gold is reduced slightly.
Taiga and Snowy Taiga:
Lapis Lazuli can be found up to layer 64, in the same "spreading" mechanic, appearing most common at layer 32. Slightly reduced diamonds.
Giant Tree Taiga:
Redstone can be found in larger quantities and up to layer 32. Slightly reduced coal and iron.
Slightly increased diamonds, can generate up to layer 24. Reduced redstone, gold and lapis lazuli.
Can generate diamonds up to layer 32. Gold only goes up to layer 16 (so you could say diamonds and gold effectively swap from what's normal). Nothing else changed.
Normal, Cold and Frozen Oceans:
Lapis Lazuli can be found up to layer 64, appearing most common at layer 32. Diamonds can be found up to layer 40, when between 16-40 slightly reduced rarity. This is to encourage exploration of underwater caves.
Warm and Lukewarm Oceans:
Redstone and Gold can be found up to layer 64.
These are all just ideas, if anyone else has any other ideas for further balancing changes feel free to let me know.