New BiomeLoadingEvent that allows modders to edit biomes as they are being deserialized.

src/main/java/net/minecraftforge/common/ForgeHooks.java

@@ -39,6 +39,8 @@ import com.google.gson.Gson;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
 import com.google.gson.JsonParseException;
+import com.mojang.serialization.codecs.RecordCodecBuilder;
+
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import net.minecraft.advancements.Advancement;
@@ -104,9 +106,15 @@ import net.minecraft.world.IBlockReader;
 import net.minecraft.world.IWorld;
 import net.minecraft.world.IWorldReader;
 import net.minecraft.world.World;
+import net.minecraft.world.biome.Biome;
+import net.minecraft.world.biome.BiomeAmbience;
+import net.minecraft.world.biome.BiomeGenerationSettings;
+import net.minecraft.world.biome.MobSpawnInfo;
 import net.minecraftforge.common.loot.IGlobalLootModifier;
 import net.minecraftforge.common.loot.LootModifierManager;
 import net.minecraftforge.common.util.BlockSnapshot;
+import net.minecraftforge.common.world.BiomeGenerationSettingsBuilder;
+import net.minecraftforge.common.world.MobSpawnInfoBuilder;
 import net.minecraftforge.event.AnvilUpdateEvent;
 import net.minecraftforge.event.DifficultyChangeEvent;
 import net.minecraftforge.event.ForgeEventFactory;
@@ -130,6 +138,7 @@ import net.minecraftforge.event.entity.player.AttackEntityEvent;
 import net.minecraftforge.event.entity.player.CriticalHitEvent;
 import net.minecraftforge.event.entity.player.PlayerEvent;
 import net.minecraftforge.event.entity.player.PlayerInteractEvent;
+import net.minecraftforge.event.world.BiomeLoadingEvent;
 import net.minecraftforge.event.world.BlockEvent;
 import net.minecraftforge.event.world.NoteBlockEvent;
 import net.minecraftforge.eventbus.api.Event.Result;
@@ -839,6 +848,21 @@ public class ForgeHooks
         throw new RuntimeException("Mod fluids must override createAttributes.");
     }
 
+    @FunctionalInterface
+    public interface BiomeCallbackFunction
+    {
+        Biome apply(final Biome.Climate climate, final Biome.Category category, final Float depth, final Float scale, final BiomeAmbience effects, final BiomeGenerationSettings gen, final MobSpawnInfo spawns);
+    }
+
+    public static Biome enhanceBiome(final ResourceLocation name, final Biome.Climate climate, final Biome.Category category, final Float depth, final Float scale, final BiomeAmbience effects, final BiomeGenerationSettings gen, final MobSpawnInfo spawns, final RecordCodecBuilder.Instance<Biome> codec, final BiomeCallbackFunction callback)
+    {
+        BiomeGenerationSettingsBuilder genBuilder = new BiomeGenerationSettingsBuilder(gen);
+        MobSpawnInfoBuilder spawnBuilder = new MobSpawnInfoBuilder(spawns);
+        BiomeLoadingEvent event = new BiomeLoadingEvent(name, climate, category, depth, scale, effects, genBuilder, spawnBuilder);
+        MinecraftForge.EVENT_BUS.post(event);
+        return callback.apply(event.getClimate(), event.getCategory(), event.getDepth(), event.getScale(), event.getEffects(), event.getGeneration().func_242508_a(), event.getSpawns().func_242577_b());
+    }
+
     private static class LootTableContext
     {
         public final ResourceLocation name;

src/main/java/net/minecraftforge/common/world/BiomeGenerationSettingsBuilder.java

@@ -0,0 +1,36 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.common.world;
+
+import net.minecraft.world.biome.BiomeGenerationSettings;
+
+import java.util.ArrayList;
+import java.util.Optional;
+
+public class BiomeGenerationSettingsBuilder extends BiomeGenerationSettings.Builder
+{
+    public BiomeGenerationSettingsBuilder(BiomeGenerationSettings orig)
+    {
+        field_242504_a = Optional.of(orig.func_242500_d());
+        orig.getCarvingStages().forEach(k -> field_242505_b.put(k, orig.func_242489_a(k)));
+        orig.func_242498_c().forEach(l -> field_242506_c.add(new ArrayList<>(l)));
+        field_242507_d.addAll(orig.func_242487_a());
+    }
+}

src/main/java/net/minecraftforge/common/world/MobSpawnInfoBuilder.java

@@ -0,0 +1,79 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.common.world;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Set;
+
+import javax.annotation.Nullable;
+
+import net.minecraft.entity.EntityClassification;
+import net.minecraft.entity.EntityType;
+import net.minecraft.world.biome.MobSpawnInfo;
+
+public class MobSpawnInfoBuilder extends MobSpawnInfo.Builder
+{
+    private final Set<EntityClassification> typesView = Collections.unmodifiableSet(this.field_242567_a.keySet());
+    private final Set<EntityType<?>> costView = Collections.unmodifiableSet(this.field_242568_b.keySet());
+
+    public MobSpawnInfoBuilder(MobSpawnInfo orig)
+    {
+        orig.getSpawnerTypes().forEach(k -> {
+            field_242567_a.get(k).clear();
+            field_242567_a.get(k).addAll(new java.util.ArrayList<>(orig.func_242559_a(k)));
+        });
+        orig.getEntityTypes().forEach(k -> field_242568_b.put(k, orig.func_242558_a(k)));
+        field_242569_c = orig.func_242557_a();
+        field_242570_d = orig.func_242562_b();
+    }
+
+    public Set<EntityClassification> getSpawnerTypes()
+    {
+        return this.typesView;
+    }
+
+    public List<MobSpawnInfo.Spawners> getSpawner(EntityClassification type)
+    {
+        return this.field_242567_a.get(type);
+    }
+
+    public Set<EntityType<?>> getEntityTypes()
+    {
+        return this.costView;
+    }
+
+    @Nullable
+    public MobSpawnInfo.SpawnCosts getCost(EntityType<?> type)
+    {
+        return this.field_242568_b.get(type);
+    }
+
+    public float getProbability()
+    {
+        return this.field_242569_c;
+    }
+
+    public MobSpawnInfoBuilder disablePlayerSpawn()
+    {
+        this.field_242570_d = false;
+        return this;
+    }
+}

src/main/java/net/minecraftforge/event/world/BiomeLoadingEvent.java

@@ -0,0 +1,131 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.event.world;
+
+import net.minecraft.util.ResourceLocation;
+import net.minecraft.world.biome.Biome;
+import net.minecraft.world.biome.BiomeAmbience;
+import net.minecraftforge.common.world.BiomeGenerationSettingsBuilder;
+import net.minecraftforge.common.world.MobSpawnInfoBuilder;
+import net.minecraftforge.eventbus.api.Event;
+import net.minecraftforge.eventbus.api.EventPriority;
+
+/**
+ * This event fires when a Biome is created from json or when a registered biome is re-created for worldgen.
+ * It allows mods to edit a biome (like add a mob spawn) before it gets used for worldgen.
+ *
+ * In order to maintain the most compatibility possible with other mods' modifications to a biome,
+ * the event should be assigned a {@link net.minecraftforge.eventbus.api.EventPriority} as follows:
+ *
+ * - Additions to any list/map contained in a biome : {@link EventPriority#HIGH}
+ * - Removals to any list/map contained in a biome : {@link EventPriority#NORMAL}
+ * - Any other modification : {@link EventPriority#LOW}
+ *
+ * Be aware that another mod could have done an operation beforehand, so an expected value out of a vanilla biome might not
+ * always be the same, depending on other mods.
+ */
+public class BiomeLoadingEvent extends Event
+{
+    private final ResourceLocation name;
+    private Biome.Climate climate;
+    private Biome.Category category;
+    private float depth;
+    private float scale;
+    private BiomeAmbience effects;
+    private final BiomeGenerationSettingsBuilder gen;
+    private final MobSpawnInfoBuilder spawns;
+
+    public BiomeLoadingEvent(final ResourceLocation name, final Biome.Climate climate, final Biome.Category category, final float depth, final float scale, final BiomeAmbience effects, final BiomeGenerationSettingsBuilder gen, final MobSpawnInfoBuilder spawns)
+    {
+        this.name = name;
+        this.climate = climate;
+        this.category = category;
+        this.depth = depth;
+        this.scale = scale;
+        this.effects = effects;
+        this.gen = gen;
+        this.spawns = spawns;
+    }
+
+    public ResourceLocation getName()
+    {
+        return name;
+    }
+
+    public Biome.Climate getClimate()
+    {
+        return climate;
+    }
+
+    public void setClimate(final Biome.Climate value)
+    {
+        this.climate = value;
+    }
+
+    public Biome.Category getCategory()
+    {
+        return category;
+    }
+
+    public void setCategory(final Biome.Category value)
+    {
+        this.category = value;
+    }
+
+    public float getDepth()
+    {
+        return depth;
+    }
+
+    public void setDepth(final float value)
+    {
+        this.depth = value;
+    }
+
+    public float getScale()
+    {
+        return scale;
+    }
+
+    public void setScale(final float value)
+    {
+        this.scale = value;
+    }
+
+    public BiomeAmbience getEffects()
+    {
+        return effects;
+    }
+
+    public void setEffects(final BiomeAmbience value)
+    {
+        this.effects = value;
+    }
+
+    public BiomeGenerationSettingsBuilder getGeneration()
+    {
+        return gen;
+    }
+
+    public MobSpawnInfoBuilder getSpawns()
+    {
+        return spawns;
+    }
+}