1.16.5

This event isn't complete and can only do basic things.

#Add

#Example script

onEvent("worldgen.add", event => {
  event.addLake(lake => { // Create new lake feature
    lake.block = "minecraft:diamond_block" // Block ID (Use [] syntax for properties)
    lake.chance = 3 // Spawns every ~3 chunks
  })

  event.addOre(ore => {
    ore.block = "minecraft:glowstone" // Block ID (Use [] syntax for properties)
    ore.spawnsIn.blacklist = false // Inverts spawn whitelist
    ore.spawnsIn.values = [ // List of valid block IDs or tags that the ore can spawn in
      "#minecraft:base_stone_overworld" // Default behavior - ores spawn in all stone types
    ]
    
    ore.biomes.blacklist = true // Inverts biome whitelist
    ore.biomes.values = [ // Biomes this ore can spawn in
      "minecraft:plains", // Biome ID
      "#nether" // OR #category, see list of categories below
    ]
    
    ore.clusterMinSize = 5 // Min blocks per cluster (currently ignored, will be implemented later, it's always 1)
    ore.clusterMaxSize = 9 // Max blocks per cluster
    ore.clusterCount = 30 // Clusters per chunk
    ore.minHeight = 0 // Min Y ore spawns in
    ore.maxHeight = 64 // Max Y ore spawns in
    ore.squared = true // Adds random value to X and Z between 0 and 16. Recommended to be true
    // ore.chance = 4 // Spawns the ore every ~4 chunks. You usually combine this with clusterCount = 1 for rare ores
  })
  
  event.addSpawn(spawn => { // Create new entity spawn
    spawn.category = "creature" // Category, can be one of "creature", "monster", "ambient", "water_creature" or "water_ambient"
    spawn.entity = "minecraft:pig" // Entity ID
    spawn.weight = 10 // Weight
    spawn.minCount = 4 // Min entities per group
    spawn.maxCount = 4 // Max entities per group
  })
})

All values are optional. All feature types have a biomes field.

Valid biome categories (#category):

taiga
extreme_hills
jungle
mesa
plains
savanna
icy
the_end
beach
forest
ocean
desert
river
swamp
mushroom
nether

You can also use $type (case doesn't matter) on Forge's BiomeDictionary:

hot
cold
wet
dry
sparse
dense
spooky
dead
lush

See BiomeDictionary for more

This is the order vanilla worldgen happens:

raw_generation
lakes
local_modifications
underground_structures
surface_structures
strongholds
undergroundores
underground_decoration
vegetal_decoration
top_layer_modification

It's possible you may not be able to generate some things in their layer, like ores in dirt, because dirt hasn't spawned yet. This means you may have to change the layer by calling ore.worldgenLayer = "top_layer_modification". But this is not recommended.

#Remove

#Example script

onEvent("worldgen.remove", event => {
  event.removeOres(ores => {
    ores.blocks = ["minecraft:coal_ore", "minecraft:iron_ore"] // Removes coal and iron ore
    ores.biomes.values = ["minecraft:plains"] // Removes it only from plains biomes
  })
  
  event.removeSpawnsByID(spawns => {
    spawns.entities.values = [
      "minecraft:cow",
      "minecraft:chicken",
      "minecraft:pig",
      "minecraft:zombie"
    ]
  })
  
  event.removeSpawnsByCategory(spawns => {
    spawns.biomes.values = [
      "minecraft:plains"
    ]
    spawns.categories.values = [
      "monster"
    ]
  })
})

If something isn't being removed, you may try to remove it "manually" by printing all features (this will spam your console a lot) and then removing them by ID where possible

onEvent("worldgen.remove", event => {
  // May be one of the decoration types/levels described in worldgen.add docs
  // But ores are *most likely* to be generated in this one
  event.printFeatures("underground_ores")
})

onEvent("worldgen.remove", event => {
  event.removeFeatureById("underground_ores", "mekanism:ore_copper")
})