Ah you mean like minigames? If so I have a fitting idea for the zombie mode :p
Cowboys vs zombies.
A team of 8 cowboys have to fight off a horde of zombies and protect their "flag" aka the base. Once a cowboy dies he will also turn into a zombie and he will be supposed to attack the cowboys like the npcs. When all cowboys or the flag health reaches 0 the game ends.
You would be able to make barricades by grabbing a crate and instead of throwing it, it would place the crate down.
Cowboys would have limited supplies of healing items/resources.
The players who turned to a zombie will receive 5 hp every hit they make.
Npc zombies would do 2 damage and player zombies 4 damage.
I think it wouldn't really fit the theme normally though, like it could fit in the theme of halloween but idk just replace some stuff with bandits and you have the same minigame