The World Tree is a concept for a single mechanic through which endless surface world can be automatically but slowly propagated outward in a void.
It starts with a World Tree sapling that can be planted on a single block of dirt. When it grows, this tree will also spread a structure of solid root blocks at and slightly below roughly surface level. Note, the preview images are not to be taken as exactly literal, I would prefer a little more randomization and rounding in the shape and size of the root network and placement of saplings.
World Tree saplings can be planted directly on root blocks as well as dirt. And the root structure, at its edges, will automatically have 1-3 saplings planted, with a 1-block deviation on the Y axis from the altitude of the origin sapling. This means that World Trees will perpetually grow and re-plant outward, gradually overtaking the void and creating a new surface world. (Ideally the auto-planting of new saplings would be able to be disabled in a config file for anyone who wants to use the mod in a non-Skyblock capacity.)
Root blocks transform over time, in certain circumstances. Under direct rainfall, root blocks will rapidly transform into grass blocks. And in very low light levels, root blocks will very slowly petrify into stone.
(The player will likely accumulate lots of root blocks, so a 1-to-1 roots to planks crafting recipe would be generous to include.)
The end result is an endlessly expanding and thickly forested surface of grass, with a small layer of stone underneath. The height variance should generate rough hills.
Other more optional features that would allow greater Skyblock functionality:
A significant chance that World Tree leaves will drop apples.
A small chance that other sapling types, cactus, cocoa beans, or sugar cane will be dropped by World Tree leaves. This could also be implemented as a chance that other sapling types spawn in the place of the World Tree saplings during structure generation.
A chance (smaller than flint from gravel) that the root block will drop potatoes, carrots, beet seeds, pumpkin seeds, or mushrooms.
A chance to generate one or two water source blocks in the bottom of the root structure.
Things that would preferably be in a config file:
Whether or not growing root blocks replace/destroy other blocks already present in the space the roots are growing into.
Whether or not World Tree saplings are automatically planted at the edges of the roots of grown World Trees (the auto-expansion).
A way to disable the above listed additional Skyblock functionality, of getting seemingly unrelated natural item drops from the World Tree blocks.
Possibly, an option to increase the number of planks craftable from World Tree wood from 4 to 6 or 8.
Possibly, a way to set the probability of apples and World Tree saplings dropping from World Tree leaves.
Possibly, a way to slow or accelerate the rate at which roots in darkness petrify into stone.
Blocks and items Recap/checklist:
World Tree sapling block. Can be planted on dirt, grass, or World Tree root blocks. Grows into World Tree structure.
World Tree wood block. Craftable into World Tree planks, 1-to-4 ratio.
World Tree leaves block. Drops World Tree saplings and apples. May also drop cocoa beans, sugar cane, cactus, oak saplings, spruce saplings, birch saplings, dark oak saplings, acacia saplings, or jungle saplings.
World Tree planks block. All regular wood planks crafting recipes should apply. For doors/boats/etc, recipe can default to the oak version of the resulting item.
World Tree root block. May drop potatoes, carrots, beetroot seeds, pumpkin seeds, red mushrooms, or brown mushrooms. Under direct rain, transforms into grass block. In low light levels, transforms into stone. Craftable 1-to-1 into World Tree planks.
This concept is free to use:
I can provide all necessary textures, if so desired. If the coder wants, I can also be a consultant on development decisions. I'm also willing to help beta-test unstable development versions of the mod as needed. But I'm basically putting the idea out there for anyone to use however they want to. I have no expectation to hold sway over any further development. However, I do expect to be told if my idea is used so I can download and use the mod myself.
If anyone has any suggestions on how ores, lava, sand, gravel, and clay could be acquired in this system, go ahead and post them below to be considered by whoever may decide to make the mod.