New world type: Cylinder world

In this world type, Overworld world generation would have the following differences:

1. The world wraps around at about x = ±12,000,000 blocks, with some margin before the Far Lands takes effect. Going east past x = +12,000,000 will transition you immediately to x = -12,000,000 (and vice versa). Additionally, there could be a visual effect added to the game to represent where the "international date line" transitions between positive and negative x, which could be nothing more complicated than permanently highlighting the north/south chunk borders with a visible black line.

2. World Borders would exist at z = ±6,000,000, or half the east/west circumference of the world.

3. Beyond some value of z that's short of the World Borders (for example, ±5,900,000), all biomes start transitioning to Ocean and Deep Ocean biomes the nearer you get to the World Border, until everything is Deep Ocean—and even underwater blocks could possibly get phased out until there's nothing but Bedrock (meaning nothing but water from y=+63 to y=-58) for the last thousand blocks before one reaches the World Border.