I had a very similar idea to this... pretty much the same. It was inspired heavily by Fallout. I was thinking of having an underground sanctuary (the vault) where the main spawn would be, and above there would be the wasteland. It would have limited free supplies along with many shops where you could buy and sell materials to help you. I tried making an (almost) pure desert world to help the whole "zombie apocalypse" theme, and found some help here:
https://forum.minetest.net/viewtopic.php?pid=103820#p103820. It really looks like a wasteland like that.
Then, you could use a mod to spawn random buildings throughout the world to finalize that wasteland feel. Various different ruins could be spawned, like half of a destroyed building, remnants of houses, and maybe even a bomb shelter - if that's possible. They could contain chests with food. With the hunger mod and bones mod, you'd need to get food from farming trees and other food from farming_plus mod to survive and not lose all your stuff. This would make the rare dirt extremely valuable.
Essential mods would be:
-firearms
-farming_plus
-teleporter
-money
-mesecons
-camo_modpack
-3d_armor
-throwing
-survival_modpack
-some sort of zombie mod like zombie waves
If someone hosted a server like this, it would probably be extremely successful!