Slime's Dungeons is my first datapack. It adds 5 structures that can be found through each of the game's dimensions, with advancements included. (Spoilers ahead!)
Spoiler
Sculk Citadel
Loot: Yes
Biome: Deep Dark
The Sculk Citadel can be found in the depths of the game. Its halls are infested with the sculk that was once sealed within it. Bring your brush, and look out for any suspicious gravel you might find.
Desert Pillars
Loot: Yes
Biome: Desert
These ominous towers loom over patterns engraved in the ancient sands near the Desert Pillars. Head to the center of the structure to descend deeper.
Primordial Caves
Loot: Yes
Biome: Nether Wastes
Ancient caverns that were formed before the characteristic lava of the Nether appeared. Inside, you can find some of the first Nether portals ever constructed.
Ender Temple
Loot: Yes
Biome: Small end islands
A confusing yet complex web of walkways, towers and rooms. These can be seen linking together End islands sometimes. They offer less loot than End cities, with less risk. Great break from the End city search!
Azalea Garden
Loot: No
Biome: Plains
The structure with the least danger. Enjoy the atmosphere and the tranquil overgrowth.
This datapack also adds a few recipes. These recipes can be found in the gallery section.
-Sculk sensor
-Sculk catalyst
-Sculk shrieker
-Crying obsidian
-Calcite