Approach to create Post apocalyptic Open world

So basically, I’d have to resize and remap textures of the world. My game is designed to have these gameplays

  1. Zombie horde (spawned and sleeping but at night will be roaming)
  2. Player + it’s vehicle
  3. Survival camps
  4. Cities and Biomes (with random resources spawn)
  5. Zombies
  6. Player’s base (base building)
    Ik this is complex that’s why I cannot sacrifice everything on world, but I also need make sure it looks visually good even with lower textures resolution. Also, level designing is hard as compared to gameplay since I’m much more of a programmer and not that creative. So I just don’t want to start without any good planning or not knowing any aspect of unreal that can help me