The Meaning of Life, the Universe, and Everything.
Idea: A biome that slowly starts overtaking EVERYTHING. Similar to how sculk can grow and take over when things die nearby, but this grows regardless. Every block could have a corrupted texture (or maybe more, like copper)((Maybe the first stage is still possible to uncorrupt a block, but after that its perminant)). Things are weaker, give poison damage, or something.
Reasons why this could be cool:
1) It could explain zombies. They are villagers that went into the Corruption and got turned
2) This could explain Ancient Cities. Trying to hide from the corruption, but ran into the Deep Dark instead.
3) Could explain why they discovered portals to the nether and End. Trying to get even further away.
It's not going to happen. The best we get to a concept of world/terrain "corruption" is the sculk. There's an important "rule" that players must have the ability to avoid or prevent griefing or world modification of any kind from other sources. It's why natural disasters (in the purest sense) don't exist in MC. That's why an infinitely-growing corruption doesn't exist in MC.
That's not to say that mods aren't an option, however -- there's likely a mod or few that are all about this kind of development. It just doesn't have a place in vanilla MC.