As 1.17 and 1.18 bring massive changes to how worldgen will be handled, Gaia should not only reflect this, but take advantage of these changes. A lot of these changes are already planned but may also be introduced in it.

Underground Structures (#10)

The plan for this long-standing issue was to populate the underground with some form of constructed features. However, due to the low priority nature of the issue, it was left to the side.

Vaults

Small structures that contain various rare treasures. It would be best to make some form of protection to these vaults, whether just spawners or a new mob, though the latter might clutter the amount of mobs necessary for the mod.

Keepsake Ruins

Slightly bigger structures made up of a few rooms with corridors. These are sorts of "locked" areas that may have been shelters from the surface. Contains various kinds of loot, mostly functional. Should look a little cosy but also worn.

Above Ground Structures

To help make the dimension seem like it may have had civilised life, some ruins or memorials would be nice to have. These just help with the overall decoration so there isn't just nature trying to make it seem populated or relying on boss structures. Gaia Minitowers already exist for some of the part, but some smaller more random features would be nice.

Memorial Structures (#53)

A small idea to try, the idea was that the biomes that would have housed civilisations had various structures to give tribute to those that had fallen. More details in the issue ticket.

Ruins

These have various sizes and shapes, but the general idea is that there was once buildings there but have since collapsed due to erosion and time. Purpose random/vague, just any sort of structural foundation will do.

New Biomes (#47, #69)

This has been considered since 1.14 but never had time to implement due to too much time allocated to porting or bug fixing. Essentially, helps to overall diversify and unify some of the biomes in Gaia as well as give some needed polish to some existing biomes.

On top of these new biomes, a new way of generating these biomes will be necessary. From the rumours of changes happening in 1.18, this might be easier to do with the concept of "biome groups". Will also help make biomes more reasonably sized and make the rare biomes a little easier to find.

All details about biome revamps and layouts will be handled in their respective tickets.

New and Improved Mobs (#42, #52, #55)

Though there are plenty of mobs as is, when new biomes come in, new mobs may be needed to help with the biodiversity. Along with this, taming and riding should be possible for battle partners or better transport over the world - especially if exploration is a necessity.

Firstly, new mobs in general. These are paired mostly with new biomes to help with the overall look and feel of the area, but some are for existing biomes for when they do get revamped.

Along with this, the rudimentary AI in most of the mobs need to be improved. These could just be the addition of a few existing AI tasks, others are just making them more smart than just walking voxels.

All the details are described in each issue.

The Elephant in the Room: Underground

Of course, the main feature of 1.17 and 1.18 is the fact that caves aren't just changing; World heights are expanded and the underground is more accommodating of various natural features, including the option to change the stone underground.

World Height

As it currently stands, these are the following levels:
Minimum height: 0
Sea Level: 63
Maximum height: 255

The proposed change would make it the following, to make Gaia seem more like a terra planet than just another overworld:
Minimum height: -64
Sea Level: TBD, between 96 and 128
Maximum height: 255

Below Y0: The Deep Underground

The general idea here is that below the original world height is now a place where primal energy and monsters that have never seen the light of day exist. The current plans here are preliminary, so ideas are bound to change.

First, this layer will be all Primal Mass. While this stone is difficult to mine, once changes are implemented, Primal Mass will be nerfed to be faster to mine. Can also add various decorative blocks, it just has that sort of potential. Keep the patches of Primal Mass ore in the lower levels of the upper Underground, because planned in the lowest level will be patches of a new kind of stone in ore veins. Block name TBD.

Next, distribution of the existing ores. I would say the current distribution is fine, no ores should change their existing level. Sugilite and Hematite will not appear in this layer. Labradorite and Moonstone will appear in the upper area. Red, Blue, and Green Opal will appear regardless of biome. Based on whatever happens with the idea of underground biomes, their distribution may vary; either certain underground biomes have certain opals or make them more frequent the lower you go. White Opal's distribution will remain roughly the same as it currently is. Pyrite Ore will appear, but they will drop more Pyrite items than the upper Underground.

Finally, mobs for the underground area. As it currently stands, the aesthetic for this new underground area is still being decided on, so mobs are currently unknown. What is known is the idea that what is underground is pure and natural, so what exists is something that lives down there rather than ventured down.