There is a life form out in space that threatens to devour the entire universe. Inspired by the infectious Phazon planets in the metroid prime series, I give you the "living planet". It's sole purpose is to grow giant spores that travel to other planets and infect them, slowly turning the entire planet into another living planet.
The most basic material that makes up all of the living planet is, for now we'll call it "corruption," but I'm sure bast could think of a better name. This corruption can exist in many forms and eats rock and metal alike to produce more corruption. This is how a planet is converted eventually. Any living thing that comes into contact with this material will ether quickly be consumed by it, or in some rare cases, transformed by it into horribly mutated, feral creatures, also known as the "turned". These creatures will then attempt to drag colonists away and expose them to corruption. Corruption also can't be stored in containers made of anything but plastic or Steel, as both aren't natural materials and can't be broken down by corruption.
The way that we would apply this to My colony is this: Create a map composed of two terrain types, ground, which consists of solid corruption, which isn't harmful to colonists and can be built on, and then for the second terrain, pools of corruption should be generated on the map. This terrain can be traversed but not built on. Corruption can be collected by corruption bots, which can't be harmed by the material, and then that corruption can be converted into regolith(rock), crystalline, gold, aluminum, and uranium. Ore must be obtained via regolith the old fashioned way. Colonists that venture into the pools of corruption are either killed immediately or turned. These turned would then seek out other colonists and drag them into the pools of corruption. The only way to stop colonists from venturing into those pools would be to put up walls around them, but that would also prevent you from harvesting the resource. Even if no colonists become turned, I would still spawn turned creatures on the map from time to time. And I would spawn more depending on the gdp of the colony. as colonists start taking from the planet, it gets angry and sends it's hordes after them(factorio inspired this idea, since you get attacked be biters for polluting the air).
In order to combat the turned, I recommend implementing security turret walls that fire at them, and a mobile bot that fires atthem as well. Bast can decide if the turned could fight back against these and damage them, but I think it would turn that planet into more of a zombie apocalypse map than an mc map if you constantly had to worry about maintaining defense. Stronger versions of the bot and turret could be added for late-gamers as well.
In order to kill corruption efficiently, you must apply ether. Otherwise corruption has to be electrified to convert it back to rock and metal, and the process is very slow. Ether and corruption annihilate each other to produce the rocks and metals once consumed by the corruption. A lot of pure energy is released as well. You can actually terraform the planet into an earth-like planet as well by injecting ether into the planet. The ether would spread towards any corruption left inside the planet and destroy it. But this would require many millions of ether and you wouldn't be able to obtain any more corruption from the planet. Certain buildings could use ether to more efficiently turn corruption into certain resources, and since pure energy is created once the two resources annihilate each other, you could make a pretty potent power plant that uses the two and generates massive amounts of regolith as a biproduct, and this could be done on other planets as well, making corruption and ether high demand resources in gbt.
This map would be a very hard map to play, possibly death level, but maybe just brutal. The most dangerous factor is the turned. If you are building ample security then you should be fine, but the don't think you can just build a huge colony without defending it, because you could get quickly overwhelmed and wiped out. Once the planet is terraformed into an ether planet, the turned die out and rivers of water should replace the pools of corruption.