I had posted this earlier but in a different section. Something like extremely rare city ruins underground. It could probably like destroy other people's computers but it would be nice. It can have things you can't on the surface maybe with new mobs or something. They could be gigantic cities that have been destroyed or buried underground. Again, it could take away a lot of memory and there might e some flaws but it sounds pretty interesting. They don't have to generate in every world and is they do generate, there's only one and they are rarer than strongholds. Tell me the flaws or your thoughts on the idea
Maybe they can be as big as a stronghold, except taller and full of mazes. Like maybe a giant ancient city. That's why they're really rare. You don't find lost cities everywhere around the world. Something players could go exploring for. It could have many new items and mobs. The bigger, the more rare. Something that you don't see In every single world, something new.