I can't code but I can give out a suggestion for a mod. It would revolve around vaults or structures spawning on the Overworld surface and when you try to
crack into them zombies and only a few skeletons would start to spawn. You could have 3 tiers of vaults with each increasing tier making the vault opening longer with more mob spawns and greater loot. If zombies are used make sure that a player can't place a villager and have all the zombies drawn towards it. Most of the inspiration for this comes from Payday 2 but seeing a mod like this would be awesome. If there is a mod like this post it below, although I am playing with forge version 1.10.2.