Is probably pretty similar to making Custom Textures.
Probably best to do it with scripting. Things to keep in mind: Interacting with mobs (E) (e.g.: Chickens) Drop Chances Textures + Animations Sounds
What are the things unique to biomes: Textures Mobs / Monsters (Mob Editor / Scripting) Different spawnrate (Biome Editor / Scripting) Dungeons / Puzzles / Secret (Level Editor) So we need mod support of all those things to make Custom Biomes happen.
Ability to swap sprite textures from anything in the game
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