When the mod updates to 1.17, I believe that vanilla Goats should be made ridable as well as being made to sometimes generate with a Dwarf riding it.
When the mod updates to 1.18, I believe that the Deepslate layer should generate in Middle Earth since it is just real life Slate, another layer of Stone. It could in the mod represent the deepest depths. There could even be some first age mobs hiding in the Deepslate area, devolved into an animal form. Perhaps Werewolf Dens and Vampire Nests could generate there to represent that other first age creatures besides Durin’s Bane went into hiding. However, if they are added, they should just be hostile on sight and not factions in order to heighten the effects that hiding for so long had on them, showing that the dangers of the depths don’t care about good vs evil. There could also be Void Fog that would generate down there as well as a new fog called Nameless Fog, which could represent the Nameless Things. This new fog would generate within Void Fog which would make it difficult to see anything while Nameless Fog would cause illusions and nausea to convey the sense of the mystery and powerlessness of the deepest depths. The horror of not knowing what might be just out of sight would be terrifying, but it does make sense to be added to represent the Nameless Things. The only thing that the Nameless Fog would not do would be dealing any damage to health.
Amethyst Geodes should generate in Middle Earth.
Vanilla Emeralds and Diamonds should generate in Middle Earth. Though Diamond weapons and armor would be disabled.
Vanilla Copper Ore should generate in Middle Earth.
Vanilla Rabbits should spawn in Middle Earth.
Bees should spawn in the Shire.
Dripstone Clusters should generate in Middle Earth alongside the mod’s Dripstones.
The Deepslate layer should generate in Middle Earth since it is just another layer of Stone, only it is Slate.
Azalea Trees should generate in Jungles.
The Dripstone Caves should generate in Middle Earth.
The Deep Dark and Wardens should generate and spawn in Middle Earth, representing the Nameless Things.
I just found an article on Tolkien Gateway that is about creatures known as Phantoms.
Phantoms: In the Elder Days, creatures of dreadful shapes, among whom are said to have been phantoms, haunted the Elves, dragging people into the darkness surrounding the borders of the Elven dwellings never to reappear.
In Beleriand, the forest of Taur-nu-Fuin was an infamous locale of phantoms of terror said to strangle or pursue lost wanderers.
Using wizardry, it was possible to summon phantoms of delusion. Sauron, master of shadows and of phantoms, devised a phantom resembling Eilinel to entrap Gorlim. In the late Third Age, Saruman was rumored to also be able to craft such phantoms.
It is also told that disembodied, phantom-like Elf-souls, from Elves who had been slain in the course of some wrong-doing, could maliciously appear before living beings to induce fear, being envious of people still alive.
Beings called phantoms also appear in the poem Oilima Markirya.
In a hastily written manuscript on small slips of paper, entitled Reincarnation of Elves, Tolkien discussed different possibilities of how the houseless fea of an Elf can have a memory so strong that it can in another fea(incarnate) induce a picture of it, so that it seems a phantom form. Such a phantom cannot be touched, but can be walked through.
Based on the etymology of qímari, linguists have suggested that Tolkien intended, in this stage of the legendarium, that a phantom was a awake...but not...living hence a mind or spirit that is incorporeal or disembodied.
In The Mod, Phantoms could have mentions in lore books and they should spawn somewhere, though I don’t know where.
I believe that the Silverfish mob in vanilla Minecraft should be able to spawn in specific places within Middle-Earth. They are in fact a real life insect.
In the mod, they should be able to spawn within dark places and ruins. Specifically they should be able to spawn within Infested Blocks in structures that include:
Ancient Harad Fortresses.
Ruined Beacon Towers.
Ruined Harnennor Settlements.
Ruined High Elven Turrets.
Ruined Dwarven Towers.
Ruined Dunedain Towers.
Ruined Gondor Towers.
Ruined Wood-elven Towers.
Small Stone Ruins.
Underwater Elven Ruins.
Ruined Overgrown Roads.