Vanilla Additions contains only a couple of features. Let's start with the biome upgrades, and the new mobs that come along with them.

This is the new flower forest biome, the new birch trees are a little larger, and uses azalea leaves. The flower forest now looks brighter, and new mobs spawn as well. The Brown Bear is one of them. Brown Bears spawn in most forest biomes including the snowy taiga forests. Brown Bears hunt salmon if they spawn near rivers. 
Brown Bears in the "Rose Gold & Revenants Update" Received new models and textures to fit the aesthetics of the add-on. Brown Bears will now hunt bees, and take less damage when stung. Brown Bears can also destroy bee hives/nests, but they can be prevented by placing down a Carved Pumpkin/Jack 'O Lantern.

The next mob native to the forest biomes is the Butterfly. These mobs take the design of the monarch butterfly, and they spawn individually in every forest biome except taiga forests and snowy taiga forests. Butterflies can be seen flying a little higher than the nearby trees. Butterflies are purely an ambient mob, so they don't have much features at the moment. 

The next mob of the forests is the Illager Patrol. The texture is still a subject to change, at the moment it's just a vindicator with pillager traits. This mob spawns in all forests, similar to the Brown Bear. They wield a special axe called the Short Axe. These illagers wander all around the world searching for trouble. You might want to be careful when going through forest biomes, these illagers deal a lot of damage if you aren't wearing any armor. 

Away from the forests, Ducks are the new mob that spawns near rivers. These mobs currently don't have their own drops, which will be added in the future. Like the Butterfly, these mobs are purely for ambience. They have a behavior of going in the water rather than staying on land. 

Now we head towards the new and improved swamp. There are new lush trees growing around the swampland, and it also makes these biomes more dangerous. Mobs can spawn during the day, seeking the tree canopies for protection. Witch huts will also fit the new environment in these new swamps. 

There are two new mobs to fit the aquatic atmosphere and the swamps. These are the new drowned mobs. The green one is called the Mossy Drowned, and the silver one is called the Deep Sea Drowned. The Mossy Drowned will spawn both in oceans and rivers/swamps, but the Deep Sea Drowned only spawns in deep oceans. 

The next mob is the Mossy Skeleton. These skeletons only spawn in jungle biomes at night. They behave identical to the skeleton. They are just a jungle biome variant of the skeleton. 

Another mob of the jungle is the Tropical Slime. These are passive creatures that jump around aimlessly. They were meant to be a bouncy way of transportation in the early stages of development, but the idea was kept for a different mob that will be added in the future.

The next two mobs spawn in the snowy tundra biome. The Frozen Slime, and the Frosted Zombie. The Frozen Slime is a hostile slime that is slower than the usual slimes that spawn underground or in swamps. And the Frosted Zombie is a biome variant of the common zombie. They effect the player with slowness when hit. 

The next mob is native to the desert. This mob is the Fennec Fox, they hunt rabbits during the night, and wander around during the day. Unlike their other biome counterparts, they don't sleep during the day since the desert doesn't have any shade. They will rest near any acacia tree if there are any nearby.

That's all the mobs that have been added currently. There are two more mobs which are the Ruby Turtle and the Cluckshroom, but those are still work in progress mobs. Now we are going to see the other features such as blocks and items.

Meet the new ore, Rubies! These ores spawn underground around Y10-0, They aren't very common at the moment, it might need some code fixes later. Rubies also have their own block, it is crafted using 9 rubies to create a block.

The next item is the Reinforced Iron Ingot. Crafted with 8 iron ingots, you can turn these into 8 Reinforced Iron Ingots that can be used t upgrade your iron sword. A Reinforced Iron Sword which deals 7 attack damage. If you'd like to spend your diamonds on a pickaxe, but still want to wield a diamond sword, you can always craft yourself a Reinforced Iron Sword. 

Here are the new bricks that have been added. Calcite Bricks, Polished Andesite Bricks, Polished Diorite Bricks, And Polished Granite Bricks. All bricks except the Calcite Bricks have a mossy brick variant. These are all crafted the same way that normal stone bricks are crafted. They are not able to be made in stone cutters at the moment. 

And for the last feature, we have the Gourdian Armor. A new set of armor that effects the player with fire resistance. This armor can be used to travel through the nether, but it requires you to explore the soulsand valley first. Using a soul campfire (and a carved pumpkin for the helmet) you can upgrade your netherite armor to something slightly stronger. This set of armor doesn't include boots, so you will get to keep your enchanted netherite boots at least. It is recommended to enchant your armor after turning your netherite armor to a Gourdian Armor set.

Update V16 Additions:

The first feature of V16 is a bit Big. Meet the Hollow Spider, a giant spider that hangs around in taiga biomes. This giant arachnid has 30 health, the good news is, they aren't always hostile. 

The next feature is a new material. The Topaz ore can be found almost everywhere underground, but currently has no use other than being a decorative material. These new ores along with the ruby will get some new features in a future update. 

Topaz Shards have one feature, and that is it can be used to make spyglasses.

Along with the other new features, the swamp has been updated. Now instead of being made of acacia wood and leaves, the new swamp tree has their own unique wood type. Currently, you can only make planks and slabs with this new wood type, but will eventually be updated with fences, doors, and trapdoors in the future. 

Here are the last three new mobs that have been added, the Green Axolotl, Coppersmith, and the Void Cube. 
The Green axolotl is just a normal axolotl, just green. They cannot breed, and instead of attacking drowned, they flee from them. Being a rare species themselves, they are protecting and avoiding danger as much as possible. 
The Void Cube is a variant of the magma cube that spawns in the end. These creatures move around slowly and will only attack if you get in their way. They are passive overall, and don't really make a threatening enemy.
And for last, the Coppersmith is a villager that spawns very rarely in the taiga biome. 

The Coppersmith has their own unique trades. They mostly do copper based items, but they also trade Topaz Shards for emeralds. 

Update V17 Additions:

The first new feature in V17, the "Rose Gold & Revenants Update" are the Soul Sentinels. These towering creatures are passive soulless beings that stare at overworld creatures out of pure curiosity. Don't get too close though! The Soul Sentinel will effect blindness which will make traversing the soulsand valley much more dangerous than it should be. (credits to Qbert for the original concept and idea, and slashbro31 for the assets)

The next new feature in V17 is a very special one. Meet the Dwelling Merchant, he's a small hooded creature with a basket full of rubies. That's right, rubies now have a proper use now, and it's used to trade goods such as banner charges, saddles, a small amount of cooked chicken, amethyst,  iron, and even cake! You could also sell rotten flesh in exchange for rubies. He also carries his best friend, which is a bee around with him during his journeys.   

There's also a new item in V17, and that's Rose Gold! Rose Gold can be crafted using gold and copper, just like Rose Gold IRL. Rose Gold can be used to craft Rose Gold Blocks, and Rose Golden Apples. The effects are Haste, Resistance, and Health Boost.
 

And for last, we got the Swamp Catfish. This mob spawns in swamp biomes, and they hunt cod and salmon if spawned near them. They drop one cod upon death, and don't flop around on land due to their large size. 
(credits to TheGibberishOne for the assets)
 

These are all the features added in Vanilla Additions so far, you can always look forward to future updates my following me on Twitter and visiting our YouTube channel. Thanks for your support!