What i mean by this is you choose two maps that you would merge (in terms of setting, atmosphere, layout, enemies, wonder weapons, aesthetic, etc)
For example: imagine the origins map but with the hellish and creepy aesthetic of mob of the dead, the robots are more worn out and have blood on their feet, bodies are everywhere, you had the staffs but with a more grimy and darker appearance, the panzer resembles more the looks of the brutus, etc
Kinda like how a few years ago, a custom map in BO3 called Wasteland did something very similar, combining the aesthetic of buried with alpha omega, having sections resembling the bunker section of alpha omega and the underground town of buried.