TLDR: The idea is in complete opposition to the Distant Horizons: A Level of Detail mod or FarPlaneTwo mod. It is a mix of Skyblock, One Chunk Survival Map with a slowly expanding world border.
Can someone make a mod, a datapack or something to make the player spawn in a slowly expanding world border ? (time adjustable, world border adjustable). I know this can be done with just 1 or 2 commands and it's pretty lame, as I've seen on other posts. But It's not what I have in mind. Picture this: Start in a normal world, random seed, but instead of generated chunks, you only start in a 2x2 blocks, kind-of-like skyblock, but the world is correctly generated based on the world seed, down to bedrock, but it's just generated only 2x2 blocks (not chunks!). The world border is set to 2x2. You can't see the blocks below the ground and can't see anything beyond the world border! After a while (time pre-settable before the game starts) the world border expands to 4x4. New blocks appear all around your tiny world. Blocks are generated accordingly to the world seed. It's just like it would have been in a normal vanilla minecraft world. The world border is now 1 block farther away making the world a column of 4x4blocks (not chunks!) starting from the bedrock to height limit. That's the only thing generated in your world. The FPS will be crazy high, obviously. No matter. As for passive or aggressive mobs they will spawn if there's proper space for them as you move up or down enough. Perhaps the world border expansion can be triggered by some advances or by player experience or something. Let's be creative about this. There's thousands upon thousands of minecraft mods out-there. Is there none to fulfill this desire?
The idea is to make the world discovery a mystery. You never know what's just 1 block beyond the world border when it will expand. Perhaps you'll see a netherrack at the surface signaling there's a ruined portal nearby building up excitement for the next expanse perhaps it will reveal the loot chest. Since the resources are so limited in such a tiny world, it makes their discovery and collecting even more rewarding. You never know what the next expanse will bring you. You must do and make with what you have pushing the player to be creative with their resources. Can you feel the immense joy finding some grass? Perhaps you'll finally start farming and put an end to eating rotten flesh. The lowest of low in terms of survival. Far greater austerity than skyblock. Can you see the dopamine reward when you get your first water source?
Can you imagine the joy when you get your first potato as a drop from your long labored mob grinder at the bottom of your tiny world? Is there no-one who shares the same dream and vision as this?
The nether will have to be generated in the same way. Will it take forever to find a fortress? Not necessary. Perhaps the player can expand the border in one direction only. Perhaps the player can exchange goods or experience or do some advances or quests or something as payment for "buying" land (generated based on world seed so an inpatient player can use Amidst to see the entire world and what direction to explore for a fortress and stronghold). Imagine revealing a 1block wide path (a vertical plane actually) in a particular direction (kind-of-like <Minecraft, but I Can Only Walk in a Straight Line challenge> but worse because you can't see anything left or right). Perhaps the border of the tiny generated world is not done by world border, but by invisible barrier blocks that just move 1 block farther based on the direction player chooses to explore. Let's be creative!