There are aspects of Middle Earth that are unsettling for being omitted.
To start with, Middle Earth outside of the Shire is a wretchedly empty and open place. The forests and woodlands seem to be few and far between, and yet, they are not filled by farms nor by pastureland. It's mostly bare plains and marshes in the places that Tolkien describes.
I find this curious, because I am from New England, and I am used to wilderness being forested. Lands abandonded by farms have not lain bare under the cold northern wind. The forest has come back. Trees have shot up around the old stone walls. Remains of brick chimneys stand amidst the wet leaves. Where a winter afternoon two hundred years ago might have been a vast white field laying open beneath the deepening blue sky, now it is deepening blue shadows stretching between the trunks, below branches that crowd over a traveller yet offer no shelter.
And yet, here is Eriador, said by Treebeard himself to have been covered in a vast forest in elder days, then to have had its woods all cut down by the men of Numenor...and in three thousand years the land has lain bare. Not a single tree has arisen in place of what came before. For all that the Old Forest is malvevolent, for all that Fangorn Forest is a witchy place, neither forest has spread. It's been three thousand years since the Numenoreans were gone and Eriador has not re-forested itself. What is the matter with the land?
From an in-universe perspective, one might assume that the emptiness is a result of the taint of Sauron. Sauron manages to taint a lot of things. (He likes to poison a situation before he moves in to strike.) And big huge wars, in Tolkien's legendarium, have a nasty habit of ruining green countrysides. The war of the Last Alliance was fought in a green place that became the Dead Marshes. The War of Wrath completely broke Beleriand. It is entirely possible that Arnor's war against Angmar would have had the same effect upon Eriador. It is also probable that while Arnor existed, its people continued the deforestation, at least until they were scattered to the winds.
From an out-of-universe perspective, the likely explanation is that, while my wilderness looks like a winter forest, Tolkien's vision of wilderness is an open moor. In England, forests are not wilderness. They are managed, they are used, they have people hunt in them. English forests are not separate from English people they way American forests are separated from American people. No, the landscape that English people call wilderness is moors and heaths. Nobody farms them, nobody builds on them. For most of English history they have been pasture.
Tolkien's vision of a land empty of people is more bleak than mine. Even bleaker than what the reality of England's ecosystem would suggest, because England is NOT different from New England in terms of how fast the forest comes back. Heaths and Moors are not truly wild in England. They are the product of grazing and burning and their current shape is due to centuries of human interaction. Yet the open lands in Middle Earth maintain themselves as open land for hundreds of years without any mention of grazing or burning! Tolkien had to have known that keeping heathlands open involved burning. He must have seen it happen. An yet, his Middle Earth is covered in vast heaths that are also neglected.
Tolkien's vision of a neglected land is one that is empty of everything, even trees. What forests there were, dwindled. Everything in Middle Earth seems to dwindle from the good old days. Elven lands diminish as their people head west. The kingdoms of humans diminish as they lose one fight after another. The forests of Middle Earth diminish and they do not recover. The guardians of the forests become tree-ish and they do not recover. Institutions in Middle Earth do not recover.
Closed systems tend to die.
That's the other unsettling thing about the legendarium. The West of Middle Earth feels like a closed system. Less so in the second and third ages when the Dunedain sailed over the wide seas...but in the First Age of the Sun, back when Beleriand was still above the waves, the Elves are never stated to trade with anyone besides the dwarves of the Blue Mountains. And they don't think of turning to anyone besides the Valar, when Morgoth comes stomping over the land. Is there anybody else on Middle Earth who they can sail to for help?
Human beings, relatively speaking, have JUST awoken. They took four hundred years to get from Hildorien to Beleriand and there's no indication that they're sophisicated enough to get messages back across the Blue Mountains. This is the unsettling background of the Silmarilion: it's wretchedly lonely. The elves and the Edain have no contact with lands beyond the Blue Mountains, no trade networks, no idea of what the world contains. They shut themselves up in their deep-delved halls and their hidden cities encircled by mountains, and they sally forth mostly to make fruitless war against Morgoth. They don't try to make it over the Blue Mountains, they don't try to contact the Avari amidst the woods of Eriador. They KEEP themselves lonely. Melian has a magic spell that keeps intruders out of the forest of Doriath, and Doriath is supposed to be the biggest Sindar settlement in Beleriand! What are they DOING in there?
Staying shut up and alone. Because besides the other kingdoms of Elves, and the domains of the Edain, there's really nobody to talk to in Middle Earth. The Quenta Silmarilion is a mythic account of the part of Middle Earth's history that's so far back that human civilization doesn't exist. It's six thousand years before the events of Lord of the Rings. Even the humans don't have trade networks yet. The Silmarilion is an account of Middle Earth's pre-history.
But why do these elves just sit there? Why do they not explore? The Silmarilion has the Noldor going back across the ice bridge to reach Beleriand, after they were perfectly willing to walk thousands of miles to reach Valinor in the west. Why did they stop in Beleriand?
And now I remember, it's the dang Silmarils. They were following the Silmarils after the Trees of Valinor were destroyed. The Elves never wanted to go back east unless it was for something that involved the west. And ever afterwards, in the long ages after the Silmarils were lost, the Elves never headed any direction but west, back home where they thought they belonged in the first place.
And this is probably why Tolkien's legendarium never explores any place east or south of Mordor. The story is about the world of the elves. How it is fading away, how it must pass so that the fourth age, the age of Man, can finally arise. Only then will Middle Earth be mapped fully. Only then will the Haradrim and the Easterlings become more important to the story than Mordor.
But it is not something the Tolkien estate will publish, because J.R.R. Tolkien had no thought for those realms. He wanted to tell a story about things diminishing, things becoming less grand than they were. He did not want to tell a story about things recovering. Many realms recover in the fourth age -- Moria, Minas Ithil, Osgiliath, Arnor -- but these are footnotes to the main point that the Elves themselves will either pass into the west or fade away. Some things you just can't get back.
Who knows. Maybe the forest of Eriador will return, or maybe Eriador will be inhabited again. As it stands in the Lord of the Rings the place feels destitute.