Should Be Defined in their own file. One line defines all biome groups. Which generates options in the rest of the Config for each entry.

In most places, such as Configuration, biomeGroups would replace Biomes: behaving identically to current biomes and contain a "biome group name" and biome list.

The exception would be SpawnLists which would still need to be done per Biome. A biomeName to list would be required to get the spawnLists for each biome grouping.

Elaborating Further:

BiomeGroup Config

• World Specific
• String defining biome groups. Use comma as delimeter.
• Seperate Category created for each entry above. Simple string to list biomeNames.
• Should probably have a list of all valid biomes somewhere inside as well.

BiomeGroup
final String groupName
final List<biomeNames>

BiomeGroupRegistry
private map(group name, biome group)
private map(biome name, List<group Names>)

addGroup(BiomeGroup group) {
  put(group.name, group)
  foreach group.biomeName
    put(biomeName, groupName)
}

LivingHandler Config

• Add a new category for biome groups the entity belongs to: :group1,group2,group3
• SpawnListEntry now use group names instead of biomeNames
• Default groups == biomeNames?

CreatureType

replace map(biomeName, List<SpawnListEntry>)
   with map(groupName, List<SpawnListEntry>)

When groups overlap, both SpawnListEntry are considered valid and retain their relative chance to spawn.