The entire world is 16 blocks wide, but infinity long
Every biome, cave, and ocean squeezed into one narrow strip running infinitely north and south. There's void separating both sides. The Nether and the End? Same strip.

Claim your stretch of the Line and reinforce with the citidel mod. Dig deep, build tall, and ride copper powered speed rails past your neighbors. And when the world gets too narrow, craft the Ant enchantment and shrink until you have more room.