A setting with “everything and the kitchen sink” - where you can believably add anything without breaking the suspension of disbelief. Typical for OSR settings.

Examples:

• Exalted: The addition of the Alchemical Exalted meant the setting could finally include both robots and dinosaurs.
• RIFTS: Dimensional rifts is the explanation for a setting where you have people with high tech equipment facing Aztec gods
• Shadowrun: It's the D&D game mixed with Cyberpunk 2020 game.