It would be cool if there were volcanoes which spits masses of lava out every 7th day or whenever. Inside the volcano there could be lava zombies. You would have to create and bring lava in to the game. I’d say lava should do half your health instantly upon contact and set you on fire. The lava should also do half the health and set on fire any other zombies that touch it apart from lava zombies. To top it all off you could have a lava zombie boss but please make the boss at least twice the normal zombie size. There also should be loot in the volcano