A number of abandoned town types that can generate alone or rarely in clusters sometimes half-buried or inside caves. Their function would be to lean into the ruins aspect of the game. Many would be made of stone brick variants. The architecture would be more advanced than villagers but still relatively medieval. Rarely coming across ruins that span a biome could create some interesting gameplay possibilities in survival and could be designed to sometimes connect with mineshafts. As a biome it might have altered spawn rates to make sure you encounter zombie variants. Flooded=drowned etc. more often than not. In terms of effect on the player it would be similar to woodland mansions. When you encounter them it is something of an event.