Fun new game modes, like a zombie outbreak, alien invaders, treasure hunt for sacred relics.

I don't want to compare other games but Sid meier's civilization 6 had a fun little game mode with a zombie outbreak, this could be used as reference. Though, to give some of my own basic ideas:
- There should be a strong emphasis on empires needing to work together to increase their changes of survival, meaning that diplomacy needs to be reworked.
- Resource system based on survival, think of water, food, clothes, bullets, gasoline, etc.

- Horde system where there is a radius of zombies and have a randomized movement pattern. Every turn, they move. How far they go and how big this radius is can be optimized later in balancing. Maybe they start off slow and small and gradually becomes bigger and faster.
- For city building, I had two ideas. Either a more nomadic play style where you can only make outposts and use them to gather resources and upon creation you have a limited amount of turns to gather your resources and head out before the horde gets to you. If they arrive while you are still in your outpost, you would have to drive them off. A battle will commence. If they get you when you are moving around in the game world, I would assume that to end in a game over. For my other idea, I was thinking of having just one capital city, and sending out units to gather resources.

For the alien invasion, I was thinking that there would a turn limit before the aliens obliterate the earth. So you either side with them or try to escape. Meaning it would be a race to either please the aliens and have them spare you or leave planet earth in time before they destroy it. So if you want to side with the aliens, you would need to fill up their ''trust meter'' through doing various tasks for them, like gathering luxurious resources, trying to sabotage/spy on other empires, etc. And if you want to escape the earth, you need to make a spaceship in time. When you start up this project, it will take longer than the countdown, so you'll have to lower the turns required for completion through various tasks.

For the sacred relic idea, I was either thinking of making this a separate game mode where you race with other empires to get all the relics and the player with the most relics wins once every relic has been collected. But this can perhaps also be implemented into the base game. Where these sacred relics are spread out across the game world, and each relic gives a buff. And they can be used as items to trade with. So for example, in the diplomacy tab you can buy a sacred relic from another player, if you really want that buff. And perhaps if you gather all sacred relics you can get a wish granted, but the sacred relics will not be functional anymore.