One thing shown at Minecraft Live was improved cave generation. Although this addon doesnt bring the new negative coordinate caves, it does try to recreate the generation. Here is an image of it:

Some biomes added to the caves were the lush caves and deep dark. Currently this addon supports lush caves and will soon add the deep dark. This is still a work in progress.

The addon also includes the mobs coming to 1.17

The warden is a mob that reacts to sound and is blind. This mob’s spawn rate has not been worked on yet, but with version 3 adding the deep dark, it will spawn correctly. The warden will vibrate when it hears a sound so you know to be careful.

Snowballs and arrows (and other projectiles) will distract the warden so pack up before fighting!

Goats have also been added along with powder snow and mountain generation. Goats will charge at you to try and knock you of the mountains and may drop a goat horn when dying, which you can interact with to make the raid sound!

The mountain generation was made by Consome. I asked if I can use it and he/she said I could.

The dripstone blocks have also been added but don’t generate yet so they are just decorative for now.

There are also more lush cave blocks that will generate soon:

Other features:

Copper

Amethyst Blocks

Non-functioning Spyglass